The world’s chemical-weapons watchdog warned Monday that Syria may have deceived experts and kept deadly chemical agents after claiming to have surrendered its entire arsenal in 2013. Ahmet Uzumcu, director-general of the Organization for the Prohibition of Chemical Weapons, questioned why traces of certain chemical weapons never declared by Syrian President Bashar al-Assad turned up in the country recently, including in a deadly sarin gas attack last year that killed dozens. Another attack believed to involve chlorine and sarin killed dozens in the rebel-held city of Douma last month. “Many Western countries are worried that the Syrian government continues to possess some of the precursors of sarin,” Uzumcu told NBC News. The OPCW collected samples from the latest attack but is still working to determine what chemicals were used. Uzumcu said some “samples collected by our experts did prove the existence of certain chemicals which were never declared.” Under a 2013 deal brokered by Russia and America, Syria agreed to destroy all of its chemical weapons.