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.