OKC Thunder Co-Owner -- Suicide Not Ruled Out ... Say Police

Oklahoma City Police say suicide has NOT been ruled out in the fiery car crash that killed OKC Thunder co-owner Aubrey McClendon ... TMZ Sports has learned.

As we reported ... McClendon died when his car smashed into a bridge at an incredible high rate of speed Wednesday morning ... hours after he was indicted on a federal conspiracy charge that carries up to a 10 year prison sentence.

We spoke with OKC Police Dept Captain Paco Balderrama who told us ... cops are working on several possible theories that led to his death -- ranging from suicide to a medical episode behind the wheel.

"Anything is possible," Capt. Balderrama said ... "At this point we just don't know."

Cops say McClendon was NOT wearing a seat belt at the time of the crash -- and he made no attempt to slow down before impact.