"He goes right up to her from behind and strong-arms her up the block," Darby said.
The man then forced the woman into her building and upstairs to her apartment, and sexually assaulted her, Darby said.
Police believe the man stole the woman's keys and cellphone before leaving the apartment. He then turned around and went back in, Darby said, possibly because he had left the messenger bag he had been carrying in the victim's apartment.
Once inside, Darby said, the man might have sexually assaulted the woman a second time.
"We believe that there's a possibility that there are several assaults that took place," Darby said. "This guy is pretty dangerous. It shows the boldness of his actions. He forces this woman into an unknown property where at any point someone could confront him."
Darby said police believe the man fled west on Spruce, then south on 20th Street. Investigators do not believe the man was carrying a weapon, Darby added.
Police were following leads and searching for additional surveillance footage Monday.
A composite sketch of the suspect, released Monday, describes the suspect as Hispanic/Mexican, 23 to 30 years old, standing between 5-foot-6 and 5-foot-9, and weighing about 150 pounds.
He is clean-shaven, with a thin build and slicked-back black hair, and at the time of the assault was wearing a black T-shirt, dark blue jeans, and a black baseball cap, police said. He carried a black messenger bag.
Anyone with information can call the Special Victims Unit at 215-685-3251, or the police tip line at 215-686-TIPS.