The White House based their report on social media users, non-governmental organizations, and other open-source outlets.
Here is the full report released by the White House on their assessment of the alleged chemical attack in Douma on April 7, 2018.
Direct link to the report:
The official report says the following:
Within hours of the first allegation of chemical use on April7, Syria’s state-run news agency painted the reports as a smear campaign by the last remaining opposition group in East Ghouta, Jaysh al-Islam.We have no information to suggest that this group has ever used chemical weapons.
Jaish al-Islam (Army of Islam) was formerly known as Liwa al-Islam (Brigade of Islam) and is a coalition of Salafist Islamist militant groups based in the Douma and Eastern Ghouta regions of the Syrian capital of Damascus.
Douma was the site of an alleged chemical weapons attack where up to 43 people were killed and it has been blamed on the Syrian regime.
But according to reports from news outlet Kurdistan 24, the group admitted it used chemical weapons in a mainly Kurdish populated area in the city of Aleppo, northern Syria in April 2016.
In an official statement, Jaish al-Islam said: “One of the field commanders in Aleppo used weapons that he was not authorised to use in these kinds of confrontations.”
So which is it?
These are the same rebels that allegedly used caged civilians as human shields. These are the people that the United States is believing? Wow. Does the Geneva convention say anything about using civilians as human shields?
Syrian rebels using caged civilian captives as ‘human shields’
Under the Statute of the International Criminal Court, “utilizing the presence of a civilian or other protected person to render certain points, areas or military forces immune from military operations” constitutes a war crime in international armed conflicts.
US says no information that these rebels have ever used chemical weapons. Really? Who knows what to believe, but there was certainly enough conflicting information available to warrant waiting to strike until after a thorough investigation.
Syrian rebels admit chemical use against Kurds
On Thursday, a group of Syrian rebels admitted it used chemical weapons against Kurds in a mainly Kurdish populated area in the city of Aleppo, northern Syria.
Jaysh al-Islam, a coalition of Islamist units involved in the Syrian civil war, said in an official statement issued on Thursday, “One of the field commanders in Aleppo used weapons that he was not authorized to use in these kinds of confrontations.”
It is worth mentioning that Mohammed Alloush, Jaysh al-Islam’s former political leader, was chosen as the chief negotiator for the Syrian opposition in Geneva.
“Jaysh al-Islam, which is killing the civilians in Sheikh Maqsoud, is included with the Syrian opposition delegation in the Geneva peace talks, while Kurds who are seeking peace and fighting terrorism are excluded from any talks related to the Syrian war,” Khalil said.
What about torture? What does the Geneva convention say about torture? Why would a rebel group with a history of using caged civilians as human shields, using chemical weapons against the Kurds, and using torture in prisons be chosen as part of a peace delegation? Sad.
He then told me that [Jaish al-Islam] is against any studies, viewpoints, universities, schools and anything else that deviates from their own way of thinking—even the Islamic colleges must teach [Jaish al-Islam’s] approach.
As for conditions inside of the prison, I was kept blindfolded and could barely see anything. I was able to discern a long, vaulted hallway and many rooms to my right and left. On the second floor was the investigations wing, which took up a large portion of the prison. However, this was all I could figure out about [that part of] the prison before being transferred to the torture wing.
In the torture wing, I walked in with my eyes blindfolded. Some of the prisoners told me that the torture was ‘the same used by the regime—there’s nothing new here. Electrical shocks, hanging you up by your limbs, tearing out your fingernails.’
I sat down and heard the sounds of torture all around me.
The truth in Douma may be challenging to figure out, but at least some sort of efforts are underway. Syria’s “…foreign ministry said it would help the Organisation for the Prohibition of Chemical Weapons (OPCW) in a fact-finding mission…” A thorough investigation would require finding out the Syrian military movements, inspecting records and interviewing armed forces personnel. It would require tracing the movements of chlorine before its release.
The anti-Assad forces in Douma have agreed to a deal in which they are leaving Ghouta and going to a place near Aleppo. “As part of the surrender deal, the Jaish al-Islam group that controlled the town released scores of people it had been holding.”
Who were these hostages? According to Wikipedia “On 1 November 2015, an opposition media outlet, Shaam News Network, posted a video showing Jaysh al-Islam militants had locked people in cages and spread out 100 cages containing about 7 captives each through Eastern Ghouta, northeast of Damascus, to use them as human shields against Syrian government air raids. According to the Syrian Observatory for Human Rights, the caged people being used as human shields were captured Alawite military officers and their families who had been kidnapped by Jaysh al-Islam two years ago outside Adra al-Ummaliyah, a government-held neighbourhood in Eastern Ghouta.” [Emphasis added].
If Jaish al-Islam was on the ropes in Douma and facing defeat in Syria, why would it not launch a false flag chemical attack, gambling, even counting, on a response from the West that would set Assad back or cripple his forces? If Assad was about to achieve the removal of anti-Assad forces from Ghouta, a project successfully underway for several weeks, why would he launch a chemical attack that would cause the West to inflict great damage on his forces?
Published by Stanford, yet the White House says in their official report that they have no information that this group has access to chemical weapons:
Jaysh al-Islam has a diverse arsenal that includes armored tanks and GRAD missiles. The group also allegedly has access to chemical weapons.   In 2013, the group had two fighter jets. 
This group is reportedly funded by Saudia Arabia.
The group operates in the embattled Syrian city of Damascus and their ongoing battle against President Bashar Al-Assad is reportedly funded by the wealthy nation of Saudi Arabia.
The Arab kingdom has sent millions of dollars to arm and train their fighters so they can defeat the Syrian regime and ‘increasingly powerful Jihadi organisations’, according to the Guardian.
Known as the Army of Islam, the insurgent group was created in 2011 when Saudi Arabia allegedly engineered the merger of over 50 rebel factions after growing alarmed at the rise if Al-Qaeda and ISIS in Syria.
h/t Daniel Higdon