Singer Jill Sobule's Death Investigated for Signs of Drugs, Alcohol

Jill Sobule's death is receiving heightened scrutiny by investigators ... because they're trying to determine when she died -- and if drugs, alcohol or carbon monoxide played any part.

A rep for the Ramsey County Medical Examiner in Minnesota tells TMZ ... investigators are digging into the cause and manner of Jill's death -- and, a major detail investigators are trying to determine is whether she was still alive when a house fire broke out, where she was later found dead.

Remember, Woodbury Public Safety received a call about an active housefire around 5:30 AM Thursday ... with first responders arriving to find the house fully engulfed in flames.

Firefighters were actively battling the blaze while searching for a missing person -- Sobule --  who homeowners believed was inside ... but, when they finally found her, Jill was already deceased.

The Ramsey County Medical Examiner rep also tells us that -- in order to figure out if she passed before the fire, or because of it -- Jill's being tested for drugs, alcohol and carbon monoxide poisoning.

Both Jill's manager and agent confirmed her death to us yesterday ... remembering Jill as a "force of nature and human rights advocate whose music is woven into our culture."

Sobule's best known for her 1995 track "I Kissed a Girl," "Supermodel" from the flick "Clueless" and the theme song to the Nickelodeon show "Unfabulous."

She was 66.