cardiopulmonary arrest...heart attack
It's sounds like anoxic brain injury, from cardiopulmonary arrest - as you say, but this wasn't from a ‘heart attack’ (occlusion of a coronary artery) in an otherwise healthy 22 year old. They either overmedicated him with something that caused him to stop breathing, or they tortured him in some way. Even water boarding, taken to the extreme, could do this if they went too long and essentially induced the equivalent of drowning. There are a ton of things they could have done to cause this, but none of them are acceptable. Did they really think he was a spy? Sick.
Cardiopulmonary arrest can come about from reasons other than a heart attack.
Overly aggresive waterboarding, drug overdose, choking, plastic bag over his head.