On August 18, almost one year after an attack that killed nearly 1,000 civilians in the rebel stronghold of eastern Ghouta, U.S. officials said the Syrian government’s chemical weapons cache had been successfully destroyed.

The process was overseen by the Organization for the Prohibition of Chemical Weapons and had been subject to numerous delays over the course of the year, requiring the transport of trucks of material through contested areas to the Latakia coast, from where they were taken out to sea.

But now a new threat has emerged with the Islamic State of Iraq and Syria (ISIS), which, just two months after beginning its Iraq offensive, has begun taking control of small chemical stockpiles.

We asked Hamish de Bretton Gordon OBE, a chemical weapons expert focusing on the Syrian crisis and founder of London-based SecureBio, to weigh in on the state of chemical weapons in Syria, one year after the Ghouta attack.

Syria Deeply: Does the regime still have chemical weapons?

Hamish de Bretton Gordon: There is a strong chance that the regime still have a small amount of chemical weapons. When the declared stock came in, it was 1300 tons of [largely] precursors. Those of us looking at it think there were 100 to 200 tons missing. In documents presented [to U.N. inspectors], there were a lot of irregularities that were commented on by Western governments. There is a general feeling that some elements, albeit fairly small, are still missing.

They also still have large stocks of chlorine, and we have seen them use that chlorine on a number of occasions, including in Hama province and in Tal Avez. And they still could use it, if they saw fit.

Why would Assad keep some of his chemical stock? It’s been good for him in battle, and the process to destroy it has kept the international community at arms length this year.

Syria Deeply: Do rebel groups like Jabhat al-Nusra, the Islamic Front or the Free Syrian Army have chemical weapons?

De Bretton Gordon: It’s very difficult to assess that. If we leave ISIS out of it, I haven’t seen any decent reporting that suggests any Syrian rebel groups have chemical weapons as they are defined by the U.N. inspectors and the international community. But they could have access to chlorine and other toxic chemicals. We have seen how dangerous chlorine has been this year, but so far its use seems to fall squarely to the regime, as it’s been dropped by helicopters – and the rebels don’t have access to helicopters.

Syria Deeply: As it pushes further north and east in Syria, and continues its offenses in Iraq, is ISIS in control of any chemicals?

De Bretton Gordon: ISIS is currently in control of a stockpile in two bunkers at the old Iraqi army barracks at al-Muthanna, about 45 miles from Baghdad. It contains remnants of Saddam Hussein’s stock – a couple thousand tons of chemical weapons. The only nerve agent there would be useless by now, but there’s a mustard gas that would still be viable. They have yet to break the bunkers open. They’ve also stolen radioactive isotopes from Mosul University in the last few weeks, and while they couldn’t make a chemical agent out of them, they could certainly use them to make dirty bombs.

Syria Deeply: When and why would they use them?

De Bretton Gordon: The key thing for which ISIS would use its chemical weapons is as a strategic tool for leverage against the international community, predominantly the U.S. There is a view, which I share, that the international community has been kept at arm’s length from Syria by the chemical weapons destruction process, and that ISIS could try the same tactic in Iraq.

ISIS could use chemicals if things started to go very badly for it on the battlefield. One of the key reasons Assad allegedly dropped sarin on Ghouta last year is because the rebels were doing well, approaching his heartland, and the chemical attack worked to beat them back. Chemical weapons work very well in the insurgency warfare that we’ve seen in Syria and now in Iraq. ISIS seemingly has no boundaries, unlike al-Qaida, which did – they developed their own chemical weapons and never used them. But ISIS won’t think twice to use chemicals if it’s a situation of last resort.