Chemical weapon investigators entered the rebel-held Douma town in Syria on Tuesday to investigate an alleged gas attack after not being allowed access for days, AFP reported. Experts from the Organisation for the Prohibition of Chemical Weapons arrived in Damascus on Saturday.

Dozens of people were reportedly killed in the chemical attack in the rebel-held town on April 7. Rescue workers said that many of the injured showed symptoms of exposure to a compound containing nerve gas. Syria and its ally Russia have denied involvement, and instead blamed rebel groups.

Soon after, the United States, United Kingdom and France ordered missile strikes against Syrian regime of Bashar al-Assad in retaliation.

On Monday, the Organisation for the Prohibition of Chemical Weapons claimed that Syrian and Russian authorities had stopped its investigators from going to the scene of the attack, blocking international efforts to establish what happened and who was to blame. France and the US have questioned the utility of an investigation as they believe Russia and Syria would have destroyed evidence by now.