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

Sources tell TMZ that Young did not leave a suicide note.  He had a diary, but it revealed nothing as to why he took his life.  The diary consists of multiple, mundane entries.

Law enforcement sources tell TMZ that authorities also went through Young's personal computer - it contained nothing revealing.

TMZ reports that members of Young's family have told authorities the actor had "depression issues" in the past.  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.

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