Other aspects of the Syrian conflict uncovered and documented by Higgins include the use of cluster bombs in 2012, which the Syrian government denied using; the proliferation of shoulder-launched heat-seeking missiles known as MANPADS; and the proliferation of Croatian-made weapons which was reportedly connected to the United States, a story later picked up by "The New York Times". He has also investigated the Syrian regime's alleged use of chemical weapons, including the Ghouta chemical attack in detail. Higgins has also performed contract work for Human Rights Watch and Action on Armed Violence. Higgins' work on who was responsible for the sarin gas attacks in the Syrian civil war was criticised in a letter to the "London Review of Books" by Theodore Postol of the Massachusetts Institute of Technology and former UN weapons inspector, Richard Lloyd.