Lee Thompson Young -- Suicide Still a Dark Mystery

Law enforcement is STUMPED as to why Lee Thompson Young shot himself to death ... TMZ has learned.

Our sources say Young did not leave a suicide note. He had a diary, but we're told it revealed nothing as to why he took his life. The diary consists of multiple, mundane entries.

Our law enforcement sources say authorities also went through Young's personal computer -- it contained nothing revealing.

We do know this. Members of Young's family have told authorities the actor had "depression issues" in the past. We're told they have the names of several doctors who may have treated Young for the problem, and authorities plan to contact the MDs to determine the severity of the depression.