EXCLUSIVE

The "mean girls" hackers who just hit the "Dance Moms" stars again have something new to fear ... the FBI.

Our "Dance Moms" sources tell us, Monday's hack was the final straw and now some cast members -- including Kelly Hyland﻿, whose two daughters were hacked, contacted the feds. We're told the FBI has actually been on the case since last month, but Monday's hack pushed it to the front burner.

We're told the FBI is not alone ... local law enforcement is also on the case.