NEW YORK - A gunman in a gas mask and a construction vest set off a smoke canister on a rush-hour subway train in Brooklyn and shot at least 10 people Tuesday, authorities said. Police were scouring the city for the shooter and a rental truck.

A scene of horror unfolded as frightened commuters ran from the train as others limped out of it. At least one collapsed on the platform.

“My subway door opened into calamity. It was smoke and blood and people screaming,” eyewitness Sam Carcamo told radio station 1010 WINS. Smoke poured out of the train car as the door opened, he added.

Five people were in critical condition but expected to survive. At least 17 in all were injured in some way in the attack that began on a subway train that pulled into a station in the Sunset Park neighborhood, about a 15-minute train ride from Manhattan and predominantly home to Hispanic and Asian communities.

Police Commissioner Keechant Sewell said that the attack was not being investigated as terrorism, but that she was “not ruling out anything.”

Authorities had a photo of a suspect and were working to confirm his identity, two law enforcement officials said. The motive remains unknown. Officers around the city were told to look out for a U-Haul truck with a certain Arizona license plate number and to stop it and detain all occupants immediately.

Investigators recovered a handgun at the scene, along with multiple smoke devices and other items they are analyzing, the officials said. They said the suspect is believed to have had at least two extended magazines.

Investigators believe the weapon jammed, preventing the suspect from continuing to fire, the officials said. The federal Bureau of Alcohol, Tobacco, Firearms and Explosives has completed an urgent trace to identify the gun’s manufacturer, seller and initial owner.

The officials were not authorized to discuss the investigation and spoke on condition on anonymity.

The attack unnerved a city on guard about a rise in gun violence and the ever-present threat of terrorism. It left some New Yorkers jittery about riding the nation’s busiest subway system and prompted officials to increase policing at transportation hubs from Philadelphia to Connecticut.

One rider’s video, shot through a closed door between subway cars, shows a person in a hooded sweatshirt raising an arm and pointing at something - possibly the door to a conductor’s booth - as five bangs sound. In another video, smoke and people pour out of a subway car, some limping.

“Someone call 911!” a person shouts.

Other video and photos from the scene show people tending to bloodied passengers lying on the platform, some amid what appear to be small puddles of blood, and another person on the floor of a subway car.

Juliana Fonda, a broadcast engineer at WNYC-FM, told its news site Gothamist she was riding the train when passengers from the car behind hers started banging on the door between them.

“There was a lot of loud pops, and there was smoke in the other car,” she said. “And people were trying to get in and they couldn’t, they were pounding on the door to get into our car.”

As police searched for the shooter, Gov. Kathy Hochul warned New Yorkers to be vigilant.

“This individual is still on the loose. This person is dangerous,” the Democrat said at news conference. “This is an active shooter situation right now in the city of New York.”

Fire and police officials were investigating reports that there had been an explosion, but Sewell said at a press conference just after noon that there were no known explosive devices. Multiple smoke devices were found on the scene, said mayoral spokesperson Fabien Levy.

After people streamed out of the train, quick-thinking transit workers ushered passengers to another train across the platform for safety, system chief Janno Lieber noted.

High school student John Butsikaris was riding the other train when he saw a conductor urging everyone to get in. He thought there might just have been a mundane problem until the next stop, when he heard screams for medical attention and his train was evacuated.

“I’m definitely shook,” the 15-year-old told The Associated Press. “Even though I didn’t see what happened, I’m still scared, because it was like a few feet away from me, what happened.”

No transit workers were physically hurt, according to their union. Besides gunshot wounds, the injured riders were treated for smoke inhalation, shrapnel and panic.

President Joe Biden, Vice President Kamala Harris and U.S. Attorney General Merrick Garland were briefed on the incident. New York Mayor Eric Adams, who is isolating following a positive COVID-19 test on Sunday, said in a video statement that the city “will not allow New Yorkers to be terrorized, even by a single individual.”

The incident happened on a subway line that runs through south Brooklyn in a neighborhood predominantly home to Hispanic and Asian communities and about a 15-minute train ride to Manhattan. Local schools, including Sunset Park High School across the street from the station, were locked down.

Danny Mastrogiorgio of Brooklyn had just dropped his son off at school when he saw a crush of passengers, some of them wounded, running up the subway stairway at the nearby 25th Street station in panic. At least two had visible leg injuries, he said.

“It was insane,” he told the AP. “No one knew exactly what was going on.”

Allan Lee was running his business, Cafe Nube, when a half-dozen police cars and fire vehicles suddenly converged on the block that contains the 36th Street station.

“Then they started ushering people that were on the block to the adjacent block and then closed off the subway entrance,” he told the AP. When he noticed bomb squad officers and dogs, he was certain it was no everyday subway problem.

New York City has faced a spate of shootings and high-profile incidents in recent months, including on the city’s subways. One of the most shocking was in January, when a woman was pushed to her death in front of a train by a stranger.

Adams, a Democrat a little over 100 days into his term, has made cracking down on crime - especially on the subways - a focus of his early administration, pledging to send more police officers into stations and platforms for regular patrols. It wasn’t immediately clear whether officers had already been inside the station when the shootings occurred.

“We say: No more. No more mass shootings. No more disrupting lives. No more creating heartbreak for people just trying to live their lives as normal New Yorkers,” Hochul said. “It has to end.”

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Associated Press reporters Michael Balsamo in Washington and Jennifer Peltz, Karen Matthews, Michelle L. Price and Julie Walker in New York contributed to this report.