Thuan started an inquiry into the usage of chemical weapons on the protestors. The investigation exonerated the Diệm regime of the most serious allegations of using poison or mustard gas. Before the president was deposed in November, the inquiry's report declared that only tear gas was used, and that the liquid components of the grenades were poured onto the protestors after they had failed to vaporize as they were designed to. A further commission chaired by General Trần Văn Đôn prior to February 1964 concluded that the tear gas was left behind by French colonial forces in the 1950s. The tear gas used came in glass containers in the form of a liquid that was transformed into gaseous vapor upon activation by acid. The injuries were attributed to the acid failing to activate the liquid into gaseous form. United States Army chemists in Maryland confirmed that the tear gas had come from canisters dating back to French World War I stocks. During World War I, France had used tear gas containing a mixture of chloroacetone and ethyl bromoacetate against German troops at Ypres on the Western Front, which was known to strongly irritate mucous membranes.