The train had crushed Ki-Suck Han in front of horrified commuters after he was shoved onto the track as the southbound Q subway pulled into the station at 49th Street near Times Square.
The assailant's image was captured on subway security cameras.
Police spokesman Paul Browne said earlier Tuesday that detectives were questioning a man who resembles the assailant after locating him near 50th Street and Seventh Avenue, not far from the station where the subway incident occurred.
Browne later said the man, 30, had "implicated himself in the incident" to detectives. Browne declined to identify the suspect. The New York Post identified the man as Nieem Davis, a street vendor from the borough of Queens.
Witnesses saw the suspect talking to himself before the altercation, leading to suspicion he may have been mentally disturbed, police said.