Authorities have a suspect in custody in the gunpoint sexual assault of a 17-year-old girl at the City Hall SEPTA station.

Shortly before 10 p.m. Tuesday, police said, a man in his 20s on the southbound platform approached the girl from behind. He took a gun from his waistband and forced her to perform a sex act before fleeing.

About 6 p.m. Wednesday, a person who resembled the suspect — who was captured on surveillance footage at the station — was stopped by SEPTA Transit Police at the 69th Street Transportation Center in Upper Darby, according to SEPTA Police Chief Thomas J. Nestel III.

The man was illegally in possession of a loaded gun, and was arrested. SEPTA said the man, whose name was not released, was being held on bail set at 10 percent of $500,000.

Capt. Sekou Kinebrew, a Philadelphia Police spokesman, said that police "strongly believe" the man committed the assault, but that investigators were still working the case. No charges had been filed in that incident as of noon Thursday.

Staff writer Chris Palmer contributed to this article.