Authorities have tightened the security detail for New York’s mayor after police uncovered “concerning” social media posts made by a suspect in the Brooklyn subway shooting.

Police were trying to track down the renter of a van possibly connected to the violence when they came upon the posts made by someone with the same name.

Chief of Detectives James Essig said investigators weren’t sure whether the man, identified as Frank R James, 62, had any link to the subway attack.

The violence had unfolded on Tuesday when a gunman in a gas mask and construction vest set off a smoke grenade and fired a barrage of bullets in a rush-hour subway train, shooting at least 10 people, authorities said.

Authorities said that the social media posts made by someone with the same name as the van renter mentioned homelessness, New York and mayor Eric Adams.

It led officials to tighten the mayor’s security detail, Mr Essig and Police Commissioner Keechant Sewell said. She said the posts were “concerning”.

The attack made the subway a scene of horror: a smoke-filled car with at least 33 rounds of gunfire going off, police said. Frightened commuters ran from the train and others limped out of it. At least one person 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.

The gunfire erupted on a train that pulled into a station in the Sunset Park neighbourhood, about a 15-minute ride from Manhattan and predominantly home to Hispanic and Asian communities.

Five people were in critical condition but expected to survive. At least 29 in all were treated at hospitals for gunshot wounds, smoke inhalation and other conditions, according to hospitals.

Ms Sewell said the attack was not being investigated as terrorism, but that she was “not ruling out anything”. The shooter’s motive was unknown.

Authorities found a nine mm semi-automatic handgun at the scene, along with extended magazines, a hatchet, detonated and undetonated smoke grenades, a black garbage can, a rolling cart, gasoline and the key to a U-Haul van, Mr Essig said.

He said the key led investigators to the van renter, finding that he has addresses in Philadelphia and Wisconsin.

The officials said authorities zeroed in on a person of interest after the credit card used to rent the van was found at the shooting scene.

The van was found, unoccupied, elsewhere in Brooklyn.

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 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.

The Metropolitan Transportation Authority announced last fall that it had put security cameras in all 472 subway stations citywide, saying they would put criminals on an “express track to justice”. But at the station where the train arrived, the cameras apparently were not working.

MTA system chief Janno Lieber told TV interviewers he did not know why the cameras malfunctioned. But he said police had “a lot of different options” from cameras elsewhere on the subway line to get a glimpse of the shooter.

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 as five bangs sound. In another video, smoke and people pour out of a subway car, some limping.