This former NFL player's death at a very young age has raised one big question

A mystery that may never be answered.

Former NFL player Keion Carpenter died Dec. 29 after he hit his head when he fell to the ground. Now, TMZ.com has obtained his autopsy report that seems to raise a big question:

What caused a seemingly healthy 39-year-old man to unexpectedly collapse?

TMZ obtained the autopsy  report that said Carpenter died of traumatic brain injuries.  The report says, "He did not make any attempts to stop his fall, striking the back of his head on the ground."

After his wife called 9-11, Carpenter regained consciousness in the ambulance and complained of a headache. He then suffered a heart attack and never recovered, TMZ reported.

The autopsy says Carpenter died of bleeding on the brain brought on by blunt force head trauma. The report notes Carpenter had hypertension but ate a healthy diet and monitored his blood pressure each day. The report also noted he suffered concussions throughout his football career, but there's been no link established between that and his fall. There was no further examination of his brain as a result of the family's request.

Carpenter, a defensive back, played six seasons with Baltimore and Atlanta between 1999 and 2005. A public viewing is set for Sunday, and funeral services, Monday.