At Mauthausen, the gas chamber played an important role in the operations of annihilation. The gas known as Zyklon-B was utilized for the first time in May of 1942, when the SS eliminated 208 Soviet prisoners of war with this system.
In the course of three years approximately 5,000 deportees where killed in the gas chamber of Mauthausen. Their bodies were burned in the annexed crematorium.
Between August 1941 and December 1944 the gas chamber of the Castle of Hartheim eliminated other sick people, those incapable of working and those from the “selections” coming not only from Mauthausen itself, but also from Gusen and Dachau.
The last mass killings took place when the war against the Nazis was near its culmination: between the 20th and the 29th of April 1945 at least 650 deportees were killed in the gas chamber, and many of these were Italians. Hundreds of other deportees – definitely numbering over 680, according to the reconstruction made by Hans Marsalek – were killed the 22nd of April 1945 with Zyklon-B gas in Barrack 31 in the satellite camp of Gusen.
The 29th of April, the SS partially bombed the gas chamber, in the effort to cancel evidence of their crimes.

For whomever wishes to know more: “The gas chamber of Mauthausen. Witness accounts and stories from Italian deportees“, by Bruno Maida