Police have charged a University Park teenager with grabbing the buttocks of a University of Maryland student who had stripped down to her underwear with two others for a late-night swim in the McKeldin Mall fountain last month, authorities said.

U-Md. police charged Robert Monroe, 18, with assault and fourth-degree sex offense by way of a criminal summons earlier this week, authorities said. He was one in a group of 8 to 10 people that heckled three female University of Maryland students who had gone for a 12:30 a.m. swim in the fountain on June 9. Police said Monroe grabbed one of the women’s buttocks before fleeing.

Police had already charged Tavon Dametrius Clark, 18, with theft and other related counts for allegedly helping to swipe the belongings of the three students. Neither he nor Monroe were U-Md. students, authorities said.

Capt. Marc Limansky, a U-Md. police spokesman, said detectives were tipped to Monroe’s involvement after interviewing suspects, witnesses and others with knowledge of the case.