 There were several reports of chemical warfare attacks, including of a nerve agent being used during the assault on Basra.Following an investigation, the United Nations (UN) found that there was no evidence that Iraq used chemical weapons to repress the uprisings, but did not rule out the possibility that Iraq could have used phosgene gas which would not have been detectable after the attack.According to the U.S. government's Iraq Survey Group, Iraqi military did in fact use the nerve agent sarin, as well as non-lethal CS gas, on a massive scale when "dozens" of improvised helicopter bombing sorties were flown against rebels in Karbala and the surrounding areas in March 1991; evidence of apparent mustard gas attacks have been also reported in the areas of Najaf and Karbala by the U.S. forces that have been stationed there at the time.