Aside from the pillow on his head, this is the classic description of a person who dies of cardiac arrhythmia during sleep.
It is NOT the description of somebody having a heart attack.
See http://healthjournalism.org/blog/2011/07/reporter-explores-dying-peacefully-in-his-sleep/
The cause for cardiac arrhythmia during sleep, with no illnesses and no recent heart surgery, is exposure to arrhythmia-inducing chemicals. Poisoning.
Ritchie also raised questions about the marshals’ actions:
“How can the Marshal say, without a thorough post mortem, that he was not injected with an illegal substance that would simulate a heart attack…”
“Did the US Marshal check for petechial hemorrhage in his eyes or under his lips that would have suggested suffocation? Did the US Marshal smell his breath for any unusual odor that might suggest poisoning? My gut tells me there is something fishy going on in Texas.”