Police mounted an intense manhunt on Tuesday for a gunman who set off a smoke bomb and opened fire in a New York subway car, injuring at least 17 people in a morning rush-hour attack that prompted new calls to fight a surge of violence in the city's transit system.

Police said the perpetrator, who was seen wearing a gas mask and was believed to have acted alone, immediately fled. The attack unfolded as a Manhattan-bound subway train on the N line of the underground rail system was pulling into a station in Brooklyn's Sunset Park neighborhood.

Ten people were hit directly by gunfire, including five hospitalized in critical but stable condition, authorities said. Seven others were injured by shrapnel or otherwise hurt in the chaos as panicked riders fled the smoke-filled subway car, some collapsing to the pavement as they poured onto the platform of the 36th Street station. The fire department said two of those hurt were treated at the scene.