Syria 'chemical' attack theories in 45 seconds
22 August 2013 Last updated at 18:50 BST
Syrian opposition activists say that more than 1,000 people were killed after government forces launched rockets with toxic agents into the Damascus suburbs in the Ghouta region early on Wednesday.
The Syrian government has described the allegations as "illogical and fabricated".
The UN has asked Syria to allow UN weapons inspectors already in country to be allowed to investigate the claims. There is no sign as yet that Damascus will allow this.
In the absence of clear evidence, the BBC's James Robbins examines who could have carried out the alleged attack.