IN THE sort of movie where global agencies are attacked by arch-villains with superpowers, the Organisation for the Prohibition of Chemical Weapons (OPCW) would make a perfect target. Its concrete and glass headquarters sits amid a cluster of high-minded international institutions in The Hague, down the street from the tribunal where Yugoslavia’s war criminals were tried and not far from the International Court of Justice. The OPCW has been busy over the past decade, destroying chemical-weapons stockpiles in Libya, Iraq and Syria, for which it won the Nobel peace prize in 2013. But the use of a nerve agent in the attempted assassination of a Russian ex-spy in Britain this month has dragged it into tricky waters.

The mission of the OPCW is to support the Chemical Weapons Convention of 1993, which bars countries from possessing them. (Using them has been illegal since The Hague Conventions of 1899, signed just across town—though that did not stop Germany and Britain from gassing each other’s troops during the first world war.) But the OPCW is not an independent enforcement agency. In theory, a country that has ratified the convention can demand an inspection on another signatory’s territory if it suspects it of stockpiling weapons, but this has never happened. Instead, the OPCW is called in only when a country agrees to eliminate its stockpiles.

The OPCW’s biggest effort to date came in Syria. In 2013, after a series of chemical-weapons attacks by the regime, Russia persuaded its ally to join the convention and eliminate its stockpiles in order to fend off America’s threat to attack. The OPCW set up a process in which weapons were transported under Russian and Chinese supervision to Norwegian and Danish ships and destroyed on board an American naval vessel. But, says Derek Chollet, an American assistant secretary of defence at the time, the agency “has no independent coercive power. It is only as powerful as countries allow it to be.”

That became clear in 2015, when chemical attacks in Syria resumed months after the OPCW confirmed that all the weapons the country admitted to possessing had been eliminated. The UN Security Council approved joint investigations that let OPCW inspectors return to Syria to find out who was at fault. But Russia disputed the inspectors’ conclusion that it was the Syrian government. When the investigations came up for renewal last November, Russia vetoed them. OPCW experts still carry out fact-finding missions in Syria after chemical attacks, but are not allowed to investigate who is to blame.

This week the OPCW sent experts at Britain’s request, to provide independent confirmation of the agent used. The British say it was the Russian nerve poison Novichok. Russia’s furious denials resemble its attacks on investigators’ conclusions in Syria, where it threw up a barrage of unconvincing challenges with little evidence.

Russia cannot block OPCW technical aid. For that it would need the support of two-thirds of the 41 countries on the organisation’s executive council. But while the agency’s experts may help determine what chemicals were used in the attack, they will not be able to lend their authority to any conclusion on the most important question: who did it?