A Ramapo College student was charged with having child pornography on his dorm-room computer after he sent sexually explicit messages to detectives posing as a prepubescent girl, authorities said Thursday.

Matthew Graichen, 24, was arrested on campus Wednesday after he "initiated sexually explicit dialogue" with investigators purporting to be a 14-year-old girl, said Bergen County Prosecutor John L. Molinelli. Investigators also found he'd been downloading child pornography on his computer, the prosecutor said.