During the Battle of the Department of Mysteries, Bellatrix hits Sirius Black with a spell in the chest. His body, still smiling, falls through the veil in the Death Chamber (OP35).
What spell does Bellatrix use to kill Sirius? Some have assumed that it's the Killing Curse. However, that curse appears as a jet of green light. The text doesn't actually say the color of the jet of light which hits Sirius, just says "the second jet of light":
Only one pair was still battling, apparently unaware of the new arrival. Harry saw Sirius duck Bellatrix's jet of red light: he was laughing at her.
'Come on, you can do better than that!' he yelled, his voice echoing around the cavernous room.
The second jet of light hit him squarely on the chest.
The laughter had not quite died from his face, but his eyes widened in shock.
Harry released Neville, though he was unaware of doing so. He was jumping down the steps again, pulling out his wand, as Dumbledore, too, turned towards the dais.
It seemed to take Sirius an age to fall: his body curved in a graceful arc as he sank backwards through the ragged veil hanging from the arch (OP35).
Since it's the "second" jet, fans assume that it's the same color as the first and therefore also red. If so, then it's clearly not Avada Kedavra, which would be green. If the second jet was actually green -- just not mentioned as such -- then it's safe to assume that it was a Killing Curse.
This brings up the question of what, then, actually caused Sirius' death. If it's not a Killing Curse, does that mean that the archway and veil are in fact a gateway to the afterlife and that simply passing through them result in death? We simply don't know.