On August 21, 2013, just after a UN team led by Professor Åke Sellström, a renowned Swedish expert on chemical warfare, had arrived in Syria to investigate the allegations of chemical weapons use, a nerve agent known as sarin was fired into the Ghouta suburbs of Damascus. The U.S. government threatened to intervene militarily to punish the government of Syrian President Bashar al-Assad, which it held to be responsible for the attack.

As the debate over intervening in Syria moved to the U.S. congress, the UN negotiated access to the Ghouta region to investigate what had happened. Sellström’s team later went on to investigate several other alleged chemical weapons attacks in Syria, including an incident in the village of Khan al-Assal west of Aleppo for which the Syrian government had blamed opposition forces.

Sellström presented his preliminary report to the UN on September 16 and a final report on December 12, giving clear evidence of the use of sarin in Syria. But by that time, events had moved on politically. Already on September 14, the United States and Russia had concluded an agreement demanding that Syria’s government hand over all its chemical weapons for destruction by the Organization for the Prohibition of Chemical Weapons (OPCW), an intergovernmental body dedicated to eliminating chemical weapons. Assad accepted and the process began—but it recently hit delays. So far, only 11 percent of the declared chemicals have been shipped out of Syria for destruction, and the future of the agreement is now increasingly in doubt. Meanwhile, the circumstances of the August 21 attack in the Ghouta area and other alleged chemical attacks in Syria continue to be hotly debated.

Today, Sellström has kindly agreed to be interviewed for Syria in Crisis. He previously served as a chief inspector of UNSCOM, the UN mission tasked with dismantling Iraq’s programs for chemical and nuclear warfare. He currently works as a project manager at the European CBRNE Center, a research institution based at Umeå University in Sweden. After Syria’s government requested a UN investigation into the Khan al-Assal attack, UN Secretary General Ban Ki-moon appointed Sellström as head of a fact-finding mission on March 26. He completed this assignment in December 2013.

Professor Sellström, it was not part of your mandate as UN investigator to identify the perpetrator of the chemical attacks in Syria. Now that you have left the UN position, do you feel that you can conclude from the total body of evidence available that the Syrian government was responsible for one or more chemical attacks?

I am not sure whether I can be a private individual on this issue, but my personal and my professional positions do not differ from each other. We do not have the evidence to say who did what, but on the other hand, we do not have the evidence to say that it could not have been done by this or that party.

You were appointed by the UN secretary general on March 26, expecting to start working in Syria within a week or so. Instead, there was a long delay before the Syrian government cleared you for entry into Syria. When you finally arrived, the August 21 attack happened. You wanted to investigate but were again forced to wait for permission. Not until Ban Ki-moon had sent his representative Angela Kane to Damascus did the Syrian government approve an investigation, which finally took place on August 26, five days after the attack. Why do you think these delays occurred?

Syria is a member of the UN. As such, it asked the UN secretary general to send a mission to investigate the alleged use of chemical weapons in Khan al-Assal on March 19, 2013. At the same time, the secretary general received allegations of use of chemical weapons in other places too, from other member states. He therefore responded that he would send a team to Khan al-Assal only if it was allowed to go to some of the other places as well. Being a sovereign state, Syria rejected the idea that other, unfriendly member states should have a say in what should or should not be investigated in the country. That is the explanation for the first delay.

The second delay, before letting us investigate the Ghouta incident, is more suggestive, although it could also have other explanations, like security risks, issues of sovereignty, or an unwillingness to break the ongoing blockade of these areas and expose their suffering to the surrounding world.

Were you able to conclude anything from the sarin used in Syria about its origins?

When you find samples of sarin, you may also find other chemicals present that may indicate some of its history. We have not been able to compare this sarin with Syrian official stores or in any other way conclude anything about is origin.

But some claim that there are indeed clues in the samples. When offering up its chemical warfare program to the UN and the OPCW for destruction, the Syrian government declared itself to be in possession of some 80 tons of hexamine, a chemical that can be used to make sarin. Your report notes hexamine traces in some of the samples from the Ghouta area. Does that mean that the sarin used on August 21 was produced by the government?

Hexamine could be a stabilizer of sarin, but others have claimed that the hexamine found could also be a contaminant being present because of the explosives.

Could these quantities of sarin have been locally produced or imported by groups other than the Syrian government?

The source for eventual nongovernmental sarin can merely be speculations at this stage—would it be synthesized or recovered from government stores or what else?

In a recent interview, you said that the death figures reported by certain opposition activists and medical personnel after the August 21 Ghouta attack were not credible. Could you explain?

The conflict in Syria is surrounded by a lot of rumors and a lot of propaganda, particularly when comes to the sensitive issue of chemical weapons.

We assess the incident in Ghouta as a relatively large incident, and I would not be surprised if hundreds of people died—we just do not know. One should realize that for every person dying, you would probably have ten people suffering from symptoms and, in addition, a number of other people who thought they had been intoxicated.

If you consider the numbers of victims intoxicated according to the reports of some opposition sources, it is just impossible that this number could have been handled in the short time available with the scarce resources available. That said, this comment should not be used to reduce or neglect or downscale the crime.

The sarin-loaded rockets analyzed by your team in the eastern Ghouta region were highly unusual, and they have been at the heart of the debate over what happened on August 21. Could you tell us something about their design and their presumed origin?

The rockets have received a lot of attention, and there is probably more information about them to be gathered outside of our investigation. They are referred to as improvised and probably otherwise intended for high explosives. Their origin is not clear at the moment.

More generally, as a scientist and as someone who saw these events up close, how did you perceive the international debate about the chemical attacks in Syria?

If you look at the statistics of the conflict, chemical weapons played a very small role. In spite of this, the use of chemical weapons in Syria became a very political issue, since it had the potential of letting some Western countries take a more active role. In the end, it may have helped to create an opening for future talks. Let’s hope for that.