The a zov battallion in mariupol. it alleges that a russian drone dropped a poisonous substance on troops and civilians in mariupol. the ukrainian defence ministry has said it s checking the reports , and the us defence department has said it cannot confirm them. let s bring in stephen rapp, who s a former international prosecutor and former us ambassador at large for global criminaljustice. welcome back. how could we know whether it is a chemical attack? how could that be verified? could that be verified? well, it would take could that be verified? well, it would take an could that be verified? well, it would take an investigation - could that be verified? well, it| would take an investigation and could that be verified? well, it - would take an investigation and when you have a mechanism to do that, the organisation for the prevention of chemical warfare, the body that enforces the chemical weapons convention of which worships a party, can go in and investigate and even under