Philadelphia police are seeking the public's help in identifying an armed intruder who assaulted a science teacher Thursday night on the campus of Drexel University.

The 6:15 p.m. incident inside a building in the 3200 block of Chestnut Street left the 57-year-old instructor "seriously shook up" but otherwise unharmed after a struggle in which the suspect swiped her cell phone as she tried to call for help, said Capt. Ben Naish, commander of Southwest Detectives.

Naish described the man as "an African American male, 20-something, 6-foot to 6-foot-3," wearing a long-sleeved T-shirt with a distinctive portrait of a mime's mask on its front side.

Investigators, using surveillance video supplied by Drexel, said the shirt appeared to be of the kind manufactured by the clothing label Affliction.

It appeared from the video that the man was casing the building or preparing to steal things, police said.

When he entered an inner office and was confronted by the teacher, "she immediately started to scream and he told her to shut up," said Naish. He grabbed her phone and fled.

Naish said that police had increased patrols in the vicinity of the campus and that Drexel officials had taken unspecified steps to heighten security.

"We are concerned because he displayed a gun," said Naish. "We'd like to get this guy off the street."

He asked anybody with information to call Southwest Detectives at 215-686-3183 or 215-686-3184.

Contact staff writer Michael Matza at 215-854-2541 or mmatza@phillynews.com.