Lisa Robin Kelly -- No Signs of Drugs, Trauma

Lisa Robin Kelly was not taking any withdrawal drugs at the time she died -- at least that's what the rehab facility told law enforcement ... TMZ has learned.

Our law enforcement sources tell us ... reps from the L.A. County Coroner and L.A. County Sheriff's homicide came to Pax Rehab House in Altadena, CA Thursday afternoon, and officials from Pax said unequivocally ... they did NOT give Lisa any drugs.

TMZ broke the story ... the star of "That '70s Show" died in her sleep Wednesday night, after being admitted to the facility on Monday for alcohol abuse.

We're told law enforcement is calling Lisa's death "a mystery." There were no drugs or alcohol in her room, no vomit in her airway, and no obvious sign of trauma.

As we reported ... Lisa's boyfriend took her to the hospital Sunday, where she registered a .34 blood alcohol level, which is borderline fatal. Lisa was admitted to Pax the next day, and she had been to rehab dozens of times before -- yes, dozens.

The Coroner is doing an autopsy.