Cops Open Investigation Into John Elway Golf Cart Accident

Cops have opened an investigation into the golf cart accident that's left John Elway's longtime friend, Jeff Sperbeck, fighting for his life, TMZ Sports has confirmed.

The Riverside County Sheriff's Office tells us ... it's started to probe the matter -- which occurred on Saturday evening in La Quinta, Calif. -- but right now, everything is "purely informational."

We're told investigators are in the process of gathering info ... and conducting interviews with witnesses. Ultimately, cops say the goal is to determine if a criminal act occurred.

The accident happened as Elway, Sperbeck, their wives, and John's son were leaving a post-Stagecoach party. Elway had been driving ... when Sperbeck suddenly fell off the back of the vehicle, hit his head, and sustained traumatic injuries.

Elway was the one who immediately dialed 911 to call for help.

Sperbeck was described as unconscious but still breathing as paramedics arrived on scene. He's now at a local hospital -- but his prognosis is not good ... we're told he's on life support -- and doctors are preparing to harvest his organs for donation.

Sperbeck worked as Elway's agent during the former NFL superstar's playing days, and the two eventually became business partners ... starting 7Cellars -- a winery -- together in 2013.