A suspected chemical weapons attack by Syrian forces on the rebel-held town of Douma has led to threats of U.S. intervention from President Trump, who in a tweet Wednesday called Syrian President Bashar al-Assad a “Gas Killing Animal who kills his people and enjoys it!”

The Syrian government and its Russian partners deny that chemical weapons were used or that Syrian forces carried out the attack, which occurred Saturday night and is reported to have killed at least 43 people.

Those claims may fall on deaf ears. In the past, Trump has implied that chemical weapons were a “red line” for him in Syria. Last year, after the apparent use of chemical weapons in the northwestern town of Khan Sheikhoun killed more than 90 people, he ordered missile strikes on a Syrian military airfield, marking the first U.S. assault on Assad's forces since the conflict began in 2011.

Trump is widely expected to pursue some kind of military action against the Syrian government again. But the red line on the use of chemical weapons in Syria has long been blurry — and big questions remain about the nature of the weapons used in Douma.

What chemical weapons have been used during the Syrian war?

The Syrian government's chemical weapons program dates to the early 1970s. Syria was thought to have one of the largest stockpiles of chemical weapons in the world, including not only nerve agents such as VX but also blister agents like mustard gas.

The most noteworthy of the chemical weapons used during the Syrian war is sarin. This nerve agent, first developed by Nazi Germany, was banned under international law in 1997. It has no smell or taste, but exposure to it quickly leads to death through asphyxiation.

The Syrian government is believed to have used sarin on a number of occasions, including the 2017 Khan Sheikhoun assault, as well as attacks in 2013 — including one on the Damascus suburbs that is thought to have killed 1,429 people, according to a White House estimate.

Chlorine gas also has been used multiple times in the war. Chlorine has industrial uses, too, and is not always considered a chemical weapon. However, if inhaled, it turns into hydrochloric acid in the lungs and can cause a person to drown from a buildup of fluids. It is less deadly than nerve agents such as sarin but can still kill.

Separately, the Organization for the Prohibition of Chemical Weapons (OPCW) has accused the Islamic State militant group of using mustard gas in attacks in Syria.

How has the United States responded to alleged chemical weapons attacks in Syria?

In 2012, President Barack Obama suggested that the use of chemical weapons in Syria could lead to U.S. intervention. “A red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized,” Obama said in unprepared remarks. “That would change my calculus.”

The next year, after Assad was suspected of using sarin in attacks, Obama threatened punitive strikes against Syria. Lacking support from Congress, he backed down.

Instead, a U.N. Security Council agreement was brokered by Moscow, as part of which Syria agreed to destroy its chemical weapons stockpiles. Assad's government acceded to the Chemical Weapons Convention on Sept. 14, 2013, technically relinquishing its stockpiles to the OPCW. The organization said it removed the last of the weapons in June 2014.

Since this agreement, there have been multiple reports of the Syrian government using chlorine gas against civilians in opposition-held areas. Chlorine gas was never included in the list of Syrian chemical weapons reported to the OPCW, although the State Department said in 2014 that its use would violate the Chemical Weapons Convention.

After the 2017 Khan Sheikhoun attack, Trump said, “That crosses many, many lines, beyond a red line.” The U.S. military soon launched 59 cruise missiles at a Syrian airfield, a largely symbolic display of might that did not cause significant damage to the Syrian military.

Although the Syrian government does not appear to have used sarin after this strike, it did not stop the use of chlorine gas. Then-Secretary of State Rex Tillerson criticized Syria for the use of chlorine gas in the rebel-held Eastern Ghouta region near Damascus early this year, but State Department officials later told reporters that this did not constitute a new “red line.”

“Even though officials have been loath to admit it, the prohibition against chemical weapons use in Syria has in the past been understood to only pertain to highly lethal nerve agents such as sarin but not the easily produced and far less deadly chlorine,” Tobias Schneider, an independent analyst tracking reported chlorine attacks, told The Washington Post in January.

Do we know what was used in Douma?

Witnesses to this weekend's bombings in Douma described a powerful smell of chlorine during the attack, but some also said that the effects of the gas appeared stronger than in previous such attacks.

Mohammed Marhoum, a medical worker, told The Post that he saw symptoms he had never seen before, including twitching, abnormal pupils and foaming at the mouth. “We believe the gas used was chlorine and another kind of gas,” he said.

Outside experts have said that the speed with which the victims died suggested that a nerve agent was used. Chlorine usually takes longer to work.

However, distinctive yellow gas cylinders have been found at the scene by activists. The open-source investigations website BellingCat noted in an article published Wednesday that these cylinders contain compressed gas. “As Sarin is a liquid a compressed gas cylinder seems an unlikely method of delivery for Sarin,” the article stated.

Experts from the OPCW are heading to the site on a fact-finding mission. It is unclear whether they will be able to gather enough evidence, as witnesses have scattered. Regardless, the investigation's outcome may not have much to do with any military option that the United States and its allies might exercise in the interim.

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