Background
This analysis is based on an older version of our inference model which is accurate but more difficult to read

Question

What caused the chemical calamity in Khan Sheikhoun on April 4, 2017?

Hypotheses
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Calculated Conclusions
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Conclusions
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1

83%
Opposition:
Opposition forces carried out a chemical attack.
83%

2

16%
Syrian Army:
The Syrian Army carried out a chemical attack.
16%

3

1.1%
Unintentional:

A Syrian Army conventional bombing unintentionally released opposition chemical agents.

1.1%

Summary

On April 4, 2017, Khan Sheikhoun, Syria came under attack. There were reports of dozens dead and hundreds injured, with those affected displaying signs of exposure to sarin.

(6th release, November 7, 2017 12:00 PM UTC):

The Rootclaim analysis finds that this was highly likely to have been a deliberate attack by the opposition. It is highly unlikely that this was a deliberate attack by the Syrian Arab Army (as claimed by the United States), or that a conventional bombing unintentionally released chemical weapons (as Russia and Syria had implied).

This revised outcome includes input from recently released HRC report 36/55 and the JIM report, and is based primarily on the following factors:

In considering the Starting Point there are hardly any useful statistics, as sarin attacks of this scale against civilians are very rare. In the most similar case (Ghouta 2013), Rootclaim's analysis found that the evidence strongly indicated it was an attack by the opposition. Key factors in that analysis were:

  1. Rocket type, impact location and direction indicated that the launch was likely from opposition controlled areas.

  2. UN laboratory findings suggested low quality, not industrially produced sarin.

  3. Leaked, likely authentic, videos depicted opposition chemical rocket launches at the same time.

  4. The opposition had used sarin on multiple occasions, one of them within days of the attack and in the same location.

Thus, the basis for the Starting Point leans moderately in favor of an opposition attack.

This inclination is strengthened when considering the further requirement that in order to carry out a chemical attack, the Syrian government would have needed to have successfully evaded the internationally supervised destruction of its chemical weapons arsenal, which was declared complete in 2015.

In evaluating the possibility of an unintentional release of opposition chemical weapons, the analysis considers the frequency of sarin use by the opposition, and the likelihood that a government attack could cause the chain of events necessary for an accidental sarin release. The result indicates that while requiring a rare coincidence, there were enough opportunities for this to occur, making it a viable option.

Subsequently, the specific evidence of this case is considered.

The key evidence:

  1. Multiple hospital records showing that many sarin victims were admitted well before the aircraft took off reduce the likelihood that the attack was carried out by aircraft.

  2. Munition remains found in a crater that is probably the sarin’s source are likely to have come from a surface to surface projectile shot from the north, which is much more likely if this was an opposition attack.

  3. Radar data only shows a Syrian plane flying a significant distance away from the crater found in Khan Sheikhoun.

  4. The government's initial reaction was to blame the poisoning on accidental release of chemicals following a conventional attack on an opposition warehouse. The timing provided for this attack was hours after the poisoning - a discrepancy that is more indicative of confusion than a cover up.

  5. The lack of photos or other supporting evidence to show that there had been a chemical weapons facility in Khan Sheikhoun, months after the attack occurred, reduces the likelihood that this was an unintended attack on a chemical weapons facility.

  6. Chemical traces from Khan Sheikhoun match traces found in previous attacks, indicating that the sarin was manually prepared, rather than accidentally being mixed following the bombing of a chemical warehouse.

  7. Chemical markers reported by the OPCW-UN Joint Investigative Mechanism are said to match the government stocks, increasing the likelihood of a government attack. This evidence is ultimately outweighed by the other factors, especially since the report provides only the positive matches, without the full chemical profile and the negative matches, which are necessary to assess the statistical significance of this finding.

  8. A Syrian / Russian attack on a hospital treating the victims and unconfirmed reports of figures associated with Syria's chemical weapons program being present that day increase the likelihood of a deliberate government attack (but are ultimately outweighed by the other factors).

Rootclaim will continue to incorporate and assess relevant evidence as it becomes available. Use the “Propose new evidence” option below to add information that is relevant to this analysis.

 

Key Evidence
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Contribution
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1
Scenario A:

Remains of an aerial sarin bomb.

Scenario B:

Remains of a surface to surface sarin projectile shot from the east, west or south.

Scenario C:

Remains of a sarin improvised explosive device (IED).

Scenario D:

Remains of something other than a sarin bomb, projectile or IED.

Scenario E:

Remains of a surface to surface sarin projectile shot from the north.

Opposition

33%

Syrian Army

67%

Unintentional

0.1%

Opposition

33%

Syrian Army

67%

Unintentional

0.1%

2
There are reports of chemical victims before 6:30 on April 4.
Opposition

67%

Syrian Army

17%

Unintentional

17%

Opposition

67%

Syrian Army

17%

Unintentional

17%

3
The morning of the chemical attack, Syrian planes flew within several kilometers of Khan Sheikhoun but not directly over it.
Sources: US radar data, Syrian pilot
The US said these were the paths of Syrian Air Force jets over Khan Sheikhoun
The US said these were the paths of Syrian Air Force jets over Khan Sheikhoun
Opposition

57%

Syrian Army

21%

Unintentional

22%

Opposition

57%

Syrian Army

21%

Unintentional

22%

4

The Syrian government acknowledged that they carried out an air strike in Khan Sheikhoun, claiming they hit a warehouse used to stockpile chemical weapons around 11:00 but implying that they did not carry out a strike at 6:30.

Sources: SANA
Opposition

52%

Syrian Army

14%

Unintentional

34%

Opposition

52%

Syrian Army

14%

Unintentional

34%

5

Videos taken in Khan Sheikhoun early that morning show four plumes of smoke which do not match the site of the crater found there, and in at least three of the sites, damage can be observed. There is a loud background noise on the video (which could be the sound of the explosions or could be the sound of planes) but there are no planes visible on the video footage.

Opposition

15%

Syrian Army

34%

Unintentional

51%

Opposition

15%

Syrian Army

34%

Unintentional

51%

Combined contribution of 20 remaining elements.
Opposition

3.5%

Syrian Army

89%

Unintentional

7%

Analysis

Evidence
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Starting Point (1 item)

Attack sites (7 items)

Two weeks after the calamity, no evidence was found of chemical material being manufactured or stored in the area of the alleged contamination zone in Khan Sheikhoun, or of any activity to indicate hiding evidence.

Warehouse next to the alleged impact site
Warehouse next to the alleged impact site
www.theguardian.com
Opposition

40%

Syrian Army

42%

Unintentional

18%

Opposition

40%

Syrian Army

42%

Unintentional

18%

A hospital in Khan Sheikhoun was partially destroyed in a bombing after the initial attack.

Sources: Syrian American Medical Society, Fadi Al-halabi
Partially destroyed Khan Sheikhoun hospital
Partially destroyed Khan Sheikhoun hospital
twitter.com
Opposition

26%

Syrian Army

50%

Unintentional

24%

Opposition

26%

Syrian Army

50%

Unintentional

24%

Ongoing bombings were making it difficult to drive in the area on the day of the attack.
Sources: Save the Children health clinic staff
Opposition

29%

Syrian Army

44%

Unintentional

27%

Opposition

29%

Syrian Army

44%

Unintentional

27%

Reports of chemical damage were confined to one part of Khan Sheikhoun.
Sources: Hadi al-Abdullah (local journalist)
Opposition

25%

Syrian Army

29%

Unintentional

45%

Opposition

25%

Syrian Army

29%

Unintentional

45%

Scenario A:

Remains of an aerial sarin bomb.

Scenario B:

Remains of a surface to surface sarin projectile shot from the east, west or south.

Scenario C:

Remains of a sarin improvised explosive device (IED).

Scenario D:

Remains of something other than a sarin bomb, projectile or IED.

Scenario E:

Remains of a surface to surface sarin projectile shot from the north.

Opposition

33%

Syrian Army

67%

Unintentional

0.1%

Opposition

33%

Syrian Army

67%

Unintentional

0.1%

Videos taken in Khan Sheikhoun early that morning show four plumes of smoke which do not match the site of the crater found there, and in at least three of the sites, damage can be observed. There is a loud background noise on the video (which could be the sound of the explosions or could be the sound of planes) but there are no planes visible on the video footage.

Opposition

15%

Syrian Army

34%

Unintentional

51%

Opposition

15%

Syrian Army

34%

Unintentional

51%

A house in Khan Sheikhoun allegedly used for weapons storage matches an attack site from that morning.
Sources: JIM report
Opposition

33%

Syrian Army

27%

Unintentional

40%

Opposition

33%

Syrian Army

27%

Unintentional

40%

Syrian Government (2 items)

The Syrian government acknowledged that they carried out an air strike in Khan Sheikhoun, claiming they hit a warehouse used to stockpile chemical weapons around 11:00 but implying that they did not carry out a strike at 6:30.

Sources: SANA
Opposition

52%

Syrian Army

14%

Unintentional

34%

Opposition

52%

Syrian Army

14%

Unintentional

34%

The Syrian government issued a clear denial of any possession of chemical weapons and denied having ever used chemical weapons.
Sources: Press TV
Opposition

34%

Syrian Army

31%

Unintentional

34%

Opposition

34%

Syrian Army

31%

Unintentional

34%

Eyewitnesses (5 items)

First responders were already using the word "sarin" before the patients had even been brought to hospitals.

Sources: Samer al-Hussein, a Syrian opposition activist
Opposition

35%

Syrian Army

33%

Unintentional

33%

Opposition

35%

Syrian Army

33%

Unintentional

33%

A nurse claimed to hear a thump but not an explosion at the time that a chemical attack allegedly occurred.

Sources: Amnesty International
Opposition

35%

Syrian Army

35%

Unintentional

30%

Opposition

35%

Syrian Army

35%

Unintentional

30%

Some eyewitnesses claimed that air strikes in Khan Sheikhoun began early in the morning on April 4, 2017, while others claimed that there were no warnings, or aircraft observed, until around 11 AM that day.
Opposition

42%

Syrian Army

29%

Unintentional

29%

Opposition

42%

Syrian Army

29%

Unintentional

29%

Around 6:30 AM, sky monitors claimed that two Su-22 planes left Homs.
Sources: Digital Forensic Research Lab
Opposition

31%

Syrian Army

34%

Unintentional

35%

Opposition

31%

Syrian Army

34%

Unintentional

35%

There were at least four reports that there was a smell associated with the attack, and at least one report there was no smell.
Zuhair Abu Abdo tells of a disgusting odor that accompanied the chemical attack
Opposition

39%

Syrian Army

22%

Unintentional

39%

Opposition

39%

Syrian Army

22%

Unintentional

39%

Syrian Air Maneuvers (3 items)

The morning of the chemical attack, Syrian planes flew within several kilometers of Khan Sheikhoun but not directly over it.
Sources: US radar data, Syrian pilot
The US said these were the paths of Syrian Air Force jets over Khan Sheikhoun
The US said these were the paths of Syrian Air Force jets over Khan Sheikhoun
Opposition

57%

Syrian Army

21%

Unintentional

22%

Opposition

57%

Syrian Army

21%

Unintentional

22%

General Mohammed Hasouri was the pilot flying near Sheikhoun around 6:30 AM.

Sources: The Times of London
Syrian pilot General Mohammed Hasouri
Syrian pilot General Mohammed Hasouri
www.thetimes.co.uk
Opposition

26%

Syrian Army

42%

Unintentional

32%

Opposition

26%

Syrian Army

42%

Unintentional

32%

Figures associated with Syria's chemical weapons program were reported to be at Shayrat airfield on April 4, 2017.
Sources: White House Press Office, The Times of London
Opposition

25%

Syrian Army

50%

Unintentional

25%

Opposition

25%

Syrian Army

50%

Unintentional

25%

United States (2 items)

An unnamed US official claims that US intelligence intercepted communications featuring Syrian military and chemical experts talking about preparations for a sarin attack in Idlib.

Sources: CNN
Opposition

32%

Syrian Army

37%

Unintentional

32%

Opposition

32%

Syrian Army

37%

Unintentional

32%

An unnamed US official claims that a Russian drone flew over a local hospital as victims were being brought to the hospital.

Sources: Associated Press
Opposition

34%

Syrian Army

34%

Unintentional

32%

Opposition

34%

Syrian Army

34%

Unintentional

32%

Chemical findings (2 items)

Laboratory study indicates that the precursor chemical DF (methylphosphonyl difluoride) used in Khan Sheikhoun contained markers consistent with the original Syrian Army stock.

Sources: JIM report
Opposition

23%

Syrian Army

53%

Unintentional

23%

Opposition

23%

Syrian Army

53%

Unintentional

23%

Diisopropyl methylphosphonate (DIMP) and hexamine traces were found in samples from Khan Sheikhoun.

Sources: Government of France
Opposition

26%

Syrian Army

37%

Unintentional

37%

Opposition

26%

Syrian Army

37%

Unintentional

37%

Other evidence (3 items)

The day before the calamity, Feras Karam tweeted that he was about to start a media campaign on chlorine attacks.

Tweet about starting a media campaign on chemical weapons attacks near Hama
Tweet about starting a media campaign on chemical weapons attacks near Hama
twitter.com
Opposition

33%

Syrian Army

33%

Unintentional

33%

Opposition

33%

Syrian Army

33%

Unintentional

33%

There are reports of chemical victims before 6:30 on April 4.
Opposition

67%

Syrian Army

17%

Unintentional

17%

Opposition

67%

Syrian Army

17%

Unintentional

17%

Khan Sheikhoun was known as a relatively pro-Syrian government town and there have been pro-government spies and informants from Khan Sheikhoun.

Sources: Masdar News
Opposition

40%

Syrian Army

33%

Unintentional

27%

Opposition

40%

Syrian Army

33%

Unintentional

27%

Storyline Assumptions
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Assumptions that derive from the evidence above are presented and evaluated here for each hypothesis.
Storyline assumptions:
Likelihood given hypothesis
(including preceding assumptions)

Discussion
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To view the discussion pages of the subanalyses, click below:
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Samuel Hapak

Apr 30, 2022 at 11:49 AM

46. With respect to identifying those responsible, the Leadership Panel has determined that the information that it has obtained constitutes sufficient credible and reliable evidence of the following:
(a) Aircraft dropped munitions over Khan Shaykhun between 0630 and 0700 hours on 4 April 2017;
(b) An aircraft of the Syrian Arab Republic was in the immediate vicinity of Khan Shaykhun between 0630 and 0700 hours on 4 April 2017;
(c) The crater from which the sarin emanated was created on the morning of 4 April 2017;
(d) The crater was caused by the impact of an aerial bomb travelling at high velocity;
(e) A large number of people were affected by sarin between 0630 and 0700 hours on the morning of 4 April 2017;
(f) The number of persons affected by the release of sarin on 4 April 2017, and the fact that sarin reportedly continued to be present at the site of the crater 10 days after the incident, indicate that a large amount of sarin was likely released, which is consistent with its being dispersed through a chemical aerial bomb;
(g) The symptoms of victims and their medical treatment, as well as the scale of the incident, are consistent with a large-scale intoxication of sarin;
(h) The sarin identified in the samples taken from Khan Shaykhun was found to have most likely been made with a precursor (DF) from the original stockpile of the Syrian Arab Republic;
(i) The irregularities described in annex II are not of such a nature as to call into question the aforementioned findings.
On the basis of the foregoing, the Leadership Panel is confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Shaykhun on 4 April 2017. The findings of the Leadership Panel regarding the evidence in this case are based on the information set forth in detail in annex II.
Do you have explanation for why have you arrived at such a different conclusions? Do you see errors in the report?

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hayan🌻حيَّانْ

Apr 10, 2022 at 2:11 PM

Free Syria from the ruthless Assad regime

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David Wooten

Apr 18, 2018 at 2:24 PM

There was no chemical attack.

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Adrian E.

Jul 2, 2018 at 7:59 PM

Wouldn't that rather be a hypothesis for the events in 2018 in Ghouta?

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Rootclaim

May 9, 2018 at 7:57 AM

The strong evidence of chemical weapon use makes this highly unlikely.

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Alexander Naffah

Apr 18, 2018 at 2:20 PM

Factions within the Syrian Army carried out an attack without executive authorisation

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Rootclaim

May 9, 2018 at 7:54 AM

Is there any evidence to support this hypothesis?

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Kent Ostby

Apr 18, 2018 at 1:02 PM

#2, #3, and #4 are based solely on your own previous data and guess in the previous attack. Therefore, you are heavily weighting this towards the opposition based on a previous guess that the opposition did something. The whole starting point is bad (and I really am only here for the math).

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Rootclaim

May 9, 2018 at 7:51 AM

As you said, this starting point is based on prior incidents as calculated in previous analyses.
However, that is not a guess; it’s a calculated conclusion based on evidence.
If there is additional information that would change the conclusions of previous analyses, the starting point for this analysis would be different.