He said the brazenness of the attack - that the assailant took her into her apartment without knowing if anyone else was at home - had made police particularly concerned.
Later Tuesday, U.S. Immigration and Customs Enforcement officials put out a statement that the agency had "lodged a detainer against" the suspect with Philadelphia police. The suspect - whom they identified as Milton Garcia-Vazquez - "was previously removed from the United States in June 2013."
The detainer allows ICE to assume custody of a suspect once local authorities are done with him.
The statement did not indicate what Garcia-Vazquez had done to merit deportation, and efforts to reach ICE officials were unsuccessful.
The victim, a 26-year-old woman, was returning to her apartment in the 1900 block of Spruce about 1:20 a.m. Saturday when a man who had been riding a bicycle approached her from behind.
Police said the assailant might have followed her after she left Ladder 15, a bar near 15th and Sansom Streets, and started walking home.
He grabbed her, saying, "Shut up, don't say anything," according to Darby.
Then, he said, the assailant forced the woman into the building, took her to her upstairs apartment, and assaulted her.
After fleeing the apartment, he may have gone back in to retrieve an item, Darby said. He added that police also believe "he attacked her a second time."
Afterward, he rode away on his bicycle with the victim's cellphone and keys, officials said.
On Saturday, police tracked a signal from her phone to the neighborhood around Eighth and Morris Streets. There, according to Lt. Anthony McFadden, who led the investigation, basic detective work eventually took them to the suspect's address in the 1800 block of South Eighth.
Detectives questioned three residents of the house and released them.
Then, after developing a sketch of the suspect with the help of the victim, police obtained a search warrant Monday and found the cellphone on the second floor of Garcia's address, McFadden said.
Detectives found her keys on the suspect when they arrested him at one of the restaurants where he works.
Darby said that Garcia made a statement to police. Along with rape, he is charged with kidnapping, burglary, robbery, and other offenses.
Tashan, one of the restaurants where Garcia worked, issued a statement saying he had been a part-time kitchen employee for three months and provided "all appropriate documents" for the job.
"Other than showing up for assigned shifts and performing his limited duties, with no customer interaction, Garcia gave us no reason to suspect he was involved in any criminal activity," according to the statement.
On Garcia's block of two-story brick rowhouses in South Philadelphia Tuesday, residents expressed surprise over the arrest.
Cristian Javier Garcia, 19, not related to the suspect, said in Spanish that he and Milton Garcia lived with seven other Honduran immigrants. Cristian Garcia said Milton Garcia worked nights in a Center City restaurant and was friends with his cousin, who also lives in the house.
Because Milton and Cristian's work shifts did not overlap, the two had limited contact. But Cristian Garcia said he considered Milton Garcia to be "good people."
He said Milton Garcia had lived at the house about a year, and his cousin had been there three years. Cristian Garcia's cousin largely oversees the three-story rowhouse.
John Ferlaino, 66, a retired forklift worker and a neighbor of Garcia's, said the suspect was "always going out at 1 or 2 in the morning on his bike."
An hour before the attack, a man with a bicycle and matching the general description of the assailant exposed himself to a 27-year-old woman at 16th and Pine Streets, police disclosed Tuesday.
Asked about that incident, Darby acknowledged that he was aware of it but did not say if investigators believed it was linked to the rape.
Darby said police are investigating to see if Garcia might be involved in other assaults. Garcia has no criminal record in Philadelphia, Darby said.
The captain said that the attacker's decision to grab the woman on the sidewalk and force her into her own apartment "showed either his boldness or desperation," Darby said. "It was a horrific crime, let's face it." He added, "It was a very unsettling, disturbing assault on a young woman."
Philly.com staff writer Michael Klein and Inquirer staff writer Chris Palmer contributed to this article.