'Unabomber' Ted Kaczynski Hanged Himself in Prison Cell...911 Audio Reveals

Ted Kaczynski, the man known as the "Unabomber," died by suicide in federal prison by hanging himself, TMZ has learned.

According to a 911 audio recording, obtained by TMZ ... someone inside the prison called emergency services Saturday morning, requesting assistance at FMC Butner in North Carolina.

The caller stated he "got a guy that was hanging ... they got a crash cart up there now." A crash cart is a wheeled container that carries medicine and equipment for use in emergency resuscitations.

Sources with direct knowledge tell us ... Kaczynski suffered serious neck injuries, including redness from ear to ear and a deformed/depressed trachea. We're told rigor mortis had also set in, particularly in his arms and legs.

One source told us leading up to his death, Kaczynski appeared ghost-like and frail -- unlike how he looked when he was arrested by the FBI in Montana on April 3, 1996. He was serving life in prison after his federal conviction for killing 3 people and maiming many others during a 20-year domestic terror campaign, involving mail bombs.

Law enforcement sources confirm to TMZ Kaczynski's death was an apparent suicide, but no official cause of death has been released pending the results of an autopsy.