It was uploaded to YouTube in 2009 and appears to be from a USHMM exhibit. The description, from a sign next to the exhibition monitor, says:
The video states:"The Nazi SS carry out a gassing experiment using human subjects at Mogilev in German-occupied Soviet Union, fall 1941. They aimed to perfect a new method for killing Jews and political enemies. The method in use - shooting people at close range - was seen as harmful to the psyche and morale of the killers. Gassing, already tested at the German "euthanasia" facilities, helped distance the killers from their victims. A German chemist, Dr. Albert Widmann, participated in this 1941 test."
I've never seen this video before. After digging deeper, these were supposedly patients from insane asylums in Mogilev and Minsk. Wikipedia quotes Arad:"To test a new killing method, hoses were run from German police vehicles into a sealed building. Psychiatric patients were brought to the building, undressed and led inside. A policeman from Einsatzgruppe B stands guard. The vehicles began running. A few minutes later, the people inside were unconscious. Both vehicles ran another 10 minutes."
Source is Arad's Belzec, Sobibor, Treblinka: The Operation Reinhard Death Camps from 1987. Not sure what Arad's sources are."In September 1941, Einsatzgruppe B was faced with the task of liquidating the patients of the lunatic asylums in the cities of Minsk and Mogilev. Nebe decided to find a simpler way for his men to kill the mentally diseased, other than by shooting them. He contacted Kripo headquarters and asked for their help in carrying out the killing of the insane with either explosives or poison gas. Dr. Widmann of the Criminal Police was sent to Nebe in Minsk, but before he left, Dr. Widmann discussed with the director of the Criminal Police Technological Institute, Dr. Heess, ways of using the carbon monoxide gas from automobile exhaust for killing operations in the East, based on the experience gained from the euthanasia program. Dr. Widmann took to Minsk 400 kg of explosive material and the metal pipes required for the gassing installations.
[...]
A few days later an experiment with poison gas was carried out by Nebe and Dr. Widmann in Mogilev. In the local lunatic asylum, a room with twenty to thirty of the insane was closed hermetically, and two pipes were driven into the wall. A car was parked outside, and one of the metal pipes that Dr. Widmann had brought connected the exhaust of the car to the pipe in the wall. The car engine was turned on and the carbon monoxide began seeping into the room. After eight minutes, the people in the room were still alive. A second car was connected to the other pipe in the wall. The two cars were operated simultaneously, and a few minutes later all those in the room were dead."
This supposedly inspired the later "gas vans". But I've never seen this footage discussed anywhere before.