The United States accused Iran of using chemical weapons as well, though the allegations have been disputed. Joost Hiltermann, the principal researcher for Human Rights Watch between 1992 and 1994, conducted a two-year study that included a field investigation in Iraq, and obtained Iraqi government documents in the process. According to Hiltermann, the literature on the Iran–Iraq War reflects allegations of chemical weapons used by Iran, but they are "marred by a lack of specificity as to time and place, and the failure to provide any sort of evidence".