Philadelphia police are asking women in West Philadelphia to keep alert to their surroundings as officers press their search for the man who assaulted two students at the University of Pennsylvania.

The assailant, who also may have attacked a Center City woman, appears familiar with the area, and in both cases managed to get into apartment buildings with security locks.

The most recent attack happened Friday night, when the assailant entered a building in the 4400 block of Spruce Street and forced the first of the two victims back inside her apartment as she was leaving. It is unclear how the attacker got into the building.

Lt. Tom McDevitt of the Special Victims Unit said women should take particular care not to let anyone in who does not have a key, and not to leave doors open even if just running a short errand.

"They should not have a false sense of security even if they are inside a locked building," McDevitt said.

City and University of Pennsylvania police have increased patrols, and also urge women to use the university's escort service or to walk with others.

In Friday's attack, the assailant encountered one of the women in the hallway about 3 p.m. and forced her at knifepoint back into her apartment. The 22-year-old victim was bound and gagged with duct tape as the assailant rummaged through the apartment, taking a laptop, cell phone, iPod and cash.

About 4:15 p.m., the victim's 24-year-old roommate arrived at the apartment. She was bound and gagged, sexually assaulted, and robbed of an iPod, cash, bank card and cell phone, police said.

The assailant is described as a goateed man in his mid- to late 20s who is just over six feet tall.

Police said the attacker resembled a man who forced a woman and her boyfriend into her apartment in the 900 block of Clinton Street on Oct. 22, bound and gagged them, then raped the woman and robbed them.