In February 1943, after being arrested, tortured and subsequently released by the Milice, the Vichy French paramilitary militia, he joined a commando group in Grenoble and then the U.S. Army Counter Intelligence Corps. When Dachau was liberated on April 29, 1945, Thomas learned the whereabouts of Emil Mahl (the "hangman of Dachau"), whom Thomas arrested two days later. Thomas, along with CIC colleague Ted Kraus, subsequently captured SS Major Gustav Knittel (wanted for his role in the Malmedy massacre). Thomas also engineered a post-war undercover sting operation that resulted in the arrest of several former SS officers. A 1950 "Los Angeles Daily News" article credits Thomas with the capture of 2,500 Nazi war criminals.