Alijah Arenas' Cybertruck ended up wrapped around a tree in an L.A. suburb early Thursday morning -- and the cause is still a mystery -- but investigators do NOT believe alcohol or drugs were a factor.

TMZ Sports spoke with our law enforcement sources ... and we're told LAPD didn't observe any signs of intoxication at the scene, just moments after the 18-year-old 5-star recruit was pulled from his burning vehicle around 4:55 AM.

After Arenas was transported to a local hospital, cops followed up ... but the soon-to-be USC basketball star was already intubated (tube down his throat), so no blood draw was done.

We're told law enforcement believes speed was the main factor in the one-car crash ... combined with a young driver and a fast vehicle (Elon's truck, depending on the model, goes 0-60 MPH in ~2.6 seconds).

Bottom line -- there's currently no criminal investigation underway.

As for Alijah's medical status, on Friday, a rep for the Arenas family told us the young hooper was making solid progress ... but was not yet out of the woods.