Justin Bieber 14-Year-Old Threatens to KILL Singer's Ex

11/8/2011 12:50 AM PST

Justin Bieber -- 14-Year-Old Threatens To Kill His Ex

The LAPD got a search warrant against someone who was after Caitlin Beadles -- Justin Bieber's former GF ... someone who wanted to skin her alive ... and the perp is a 14-year-old girl.

According to the warrant -- obtained by TMZ -- someone went on tumblr.com, a social media website, using the name KILLSGANDCB -- we're guessing it means kill Selena Gomez and Caitlin Beadles.   The perp said she was going to "murder" Beadles and had cameras trained on her 24/7, ominously promising the victim won't know what's coming until she "wakes up on the other side of her bed with a knife."

TMZ has learned ... the suspect is a 14-year-old girl who lives in Kentucky who is developmentally disabled.  We're told LAPD detectives contacted her mother, who is now closely monitoring her Internet activities.

Meanwhile, Selena's alleged stalker, Thomas Brodnicki, remains in jail on felony stalking charges and cops say he's a real threat to her.