Labels of the Organisation for the Prohibition of Chemical Weapons (OPCW) are seen iside a damaged house in Douma in Damascus, Syria April 23, 2018. REUTERS/ Ali Hashisho
<p>Labels of the Organisation for the Prohibition of Chemical Weapons (OPCW) are seen inside a damaged house in Douma in Damascus, Syria April 23, 2018. Ali Hashisho&nbsp;/ Reuters</p>

Douma evidence may have 'slipped away' because of Syrian and Russian delays