Official on U.N. commission says Syrian rebel group may have used nerve agent . Peter Bergen : It would not be first time al Qaeda-affiliated group used chemical weapons . The U.S. and its allies blame the Assad regime for using sarin gas against rebels last year, but Russia has said it is investigating whether militants from that opposition were behind a recent attack outside Damascus. Bergen : Issue of who may have used them in Syria is key to U.S. response .
Official on U.N. commission says Syrian rebel group may have used nerve agent . Peter Bergen : It would not be first time al Qaeda-affiliated group used chemical weapons . Obama administration raises doubts that rebels used chemical weapons . Bergen : Issue of who may have used them in Syria is key to U.S. response .