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Hollywood cops and prosecutors want to go after a guy who flew a drone over the Hollywood police dept., but we've learned they're stumped.

Law enforcement sources tell TMZ ... several people have become a thorn in the side of the LAPD -- trying to expose what they claim are police misdeeds. One of them flew a drone over the Hollywood Division Tuesday afternoon, shooting video of the parking lot with prisoners and undercover officers.

The parking lot is shielded by a wall for security reasons -- so it's not visible from the street.

We're told police detectives and lawyers from the L.A. County D.A. and the L.A. City Attorney had a meeting to figure out what criminal laws might have been violated, but they concluded as long as the drone flies lower than 400 feet ... there's nothing they can do. Anything above is covered by the FAA.

One law enforcement source tells TMZ ... they're worried about paparazzi flying over the backyards of celebs and shooting them by the pool or doing various activities.