In May 1940, Hansgirg left Japan for the United States after the political mood became "hostile to foreigners". In the United States, Hansgirg joined forces with American industrialist Henry J. Kaiser. Before the war, the demand for aircraft engineering had increased the need for light magnesium alloys. Kaiser had paid $750,000 to Winter for the patent in the carbothermic process and received government credits of about $22 million to build a plant for Permanente Metals Corporation (PMC) in California By 1941, the first unit at Kaiser's plant was producing about  of magnesium per day.
However, getting the Permanete plant working was dangerous and fraught with problems. The technology for the Hansgirg process was still as unreliable as it had been in Radenthein, as it required handling magnesium dust which ignites explosively in air. PMC engineers built a pilot plant to improve the process. However, instead of modifying the approach, PMC chemists found a new use for the magnesium dust. They invented a new incendiary mixture of "goop" – a paste of magnesium dust gelled with gasoline and additives, which could be used as a bomb. Government controls allowed PMC to price the "goop" in competition with napalm, and avoid large financial losses. By September 1943, only 10% of the dust was being used to produce magnesium as ingots. For the entire war period, PMC issued 20 million lbs. of magnesium and 86 million lbs. of "goop". The proportion of incendiary bombs with "goop" constituted about 8% of the total tonnage of incendiary devices used in the bombing of Japan and Germany. After the war, the Permanente plant was halted, and at the end of 1945 Kaiser repaid his credits.
Nine days after the December 7, 1941, attack on Pearl Harbour plunged the United States into World War II, Hansgirg was arrested by the FBI on a presidential warrant accused of being "potentially dangerous to the public peace and safety of the United States".
After arrest, Hansgirg was first held in jail in Santa Clara County, California, and later at U.S. alien internment camps in San Antonio, Texas and Stringtown, Oklahoma.
During Hansgirgs wartime internment, U.S. attorney general Francis Biddle denied permission for his wife to visit him. Heller appealed in a hand-written woman-to-woman' letter to First Lady Eleanor Roosevelt seeking leniency, explaining that her husband could not criticize Adolf Hitler because their son was still in the German army and would be retaliated against if he did so. She ended this letter by stating: