The Department of Mysteries is a place where the Ministry of Magic investigates into various mysteries of life (and death!). There are rooms for cognition, time, prophecy, death and more. In one of the rooms, they keep the Veil.
Who is responsible for Sirius Black's death?
Obviously, Bellatrix Lestrange didn't kill him because the spell wasn't Avada Kedavra. The veil is to be blamed. I read that the veil separates the world of the living from that of the dead. So, Sirius fell to his death. He moved beyond the veil. But the veil wouldn't have been there if it hadn't been for the mad experiments and meddlings with death that the Ministry conducted.
So, is the Ministry responsible for Sirius's death? After the Dark Lord was defeated [or destroyed], did they stop messing with things that could take lives just like that?