3:17 PM PT -- Cops now suspect Salaam's death was suicide and say a note has been recovered at the scene.

Officials have still not released the cause of death.

Ex-Heisman Trophy winner Rashaan Salaam -- a superstar running back for the Colorado Buffaloes -- has died at age 42 ... his family confirmed.

Officials tell TMZ Sports ... Salaam's body was discovered in a parking lot at Eben G. Fine Park in Boulder, Colorado late Monday night.

We're told there were no obvious signs of foul play. An autopsy is scheduled to be performed.

Salaam won the Heisman in 1994 -- beating out guys like Steve McNair, Warren Sapp and Kerry Collins.

He was later drafted by the Bears in the 1st round of the 1995 Draft but only spent a few seasons in the NFL.

Salaam went on to run an organization dedicated to helping underprivileged kids.