Rebels seized control of Syria’s capital on Sunday, ending the rule of Bashar al-Assad, who used chemical weapons to kill thousands of his own citizens during the country’s 13-year civil war.

Now, a militia group that once had ties to Islamic State and al-Qaeda before disavowing them in 2016 controls Damascus and possibly what’s left of Assad’s arsenal, raising concerns in the United States and other countries that chemical weapons could fall into the wrong hands.

What is Syria’s history with chemical weapons?

Assad’s use of chemical weapons on rebel groups and civilians characterized the early days of Syria’s civil war, so much so that in 2012, then-US President Barack Obama warned that their continued use would cross a “red line” that would justify US military intervention.

Assad ignored the warning and on August 21, 2013, launched a sarin gas attack on Ghouta, a suburb near Damascus, killing more than 1,400 civilians, including hundreds of children.

Under the threat of US retaliation, Assad agreed to a deal brokered by the US and Russia to eliminate his country’s chemical weapons program, which he had until then denied, and join an international treaty banning chemical weapons .

The Organization for the Prohibition of Chemical Weapons, an international watchdog, was tasked with destroying Syria’s chemical weapons stockpile in 2013, an operation for which it won that year’s Nobel Peace Prize.

Over the next nine months, the OPCW destroyed approximately 1,100 metric tons of sarin, VX, and mustard gas and their delivery mechanisms, and certified, in June 2014, that all of Syria’s declared weapons had been removed.

Why is there concern?

The OPCW’s certification, however, stated that the chemical weapons “declared” by Syria were destroyed.

But the census of Syria was not complete.

Even at the time, US and OPCW officials suspected that Assad had hidden some of his stockpiles and chemical weapons facilities from inspectors, a suspicion that was confirmed three years later when more than 80 civilians were killed in a bombardment with chemicals by Syrian forces in Khan Sheikhoun.

“We always knew that we hadn’t destroyed everything, that the Syrians weren’t being truthful in their statement,” Anthony Blinken, deputy secretary of state under President Obama, told the New York Times at the time.

Almost exactly a year after the Khan Sheikhoun attack, around 50 more people were killed on April 7, 2018, in another chemical attack near Damascus.

The United States, Britain and France responded by bombing three government chemical weapons storage and research facilities near Damascus. However, US Defense Department officials suspected that the Syrian government may have retained some of its ability to use chemical weapons.

How many chemical weapons are left and where are they now?

As recently as last week, US intelligence agencies were closely monitoring suspected chemical weapons storage sites in Syria, looking for signs that government forces might be preparing to use the remaining stockpiles to prevent rebels from taking the capital. Now that the Assad government has fallen, there is concern that the weapons could be stolen or used.

On December 9, Israel confirmed it had carried out airstrikes on suspected chemical weapons and missile stockpiles in Syria to prevent the weapons from falling “into the hands of extremists,” Israeli Foreign Minister Gideon Sa’ar said.

Ralph Trapp, an independent adviser on chemical and bioweapons disarmament who has closely followed the development of Syria’s chemical weapons program and its destruction, said he suspects that what remains of the government’s chemical weapons arsenal is small.

How easily could chemical weapons be used?

Hard. Chemical weapons are not stored in a ready-to-use form. The chemicals for sarin gas can be combined in the warhead only shortly before deployment — hours or days at most, Trump said.

“If they don’t know how to mix it together and how to do it safely without killing themselves, I would be skeptical about an armed group,” he added.

What can be done to keep the arsenal safe?
The OPCW said on Monday that it is closely monitoring recent developments in Syria, as well as the security and integrity of all of the country’s declared chemical weapons research, development, production, storage and testing sites. Once a new government is in place, Trump said the OPCW will try to work with the new leadership to destroy what’s left.