The search focused partly on a man who police say rented a van possibly connected to Tuesday's violence.
Investigators stressed they weren't sure whether the man, Frank R. James, was responsible for the shooting.
But authorities were examining social media videos in which the 62-year-old decried the United States as a racist place awash in violence and sometimes railed against New York City Mayor Eric Adams.
"This nation was born in violence, it's kept alive by violence or the threat thereof and it's going to die a violent death. There's nothing going to stop that," James said in one video.
Police Commissioner Keechant Sewell called the posts "concerning" and officials tightened security for Adams, who was already isolating following a positive COVID-19 test.
Adams on Wednesday said investigators were zeroed-in on finding James.
"We are going to continue to close the loop around him and bring him in, and continue the investigation into this horrific act against innocent New Yorkers," the Democrat said on MSNBC.
The gunman sent off smoke grenades in a crowded subway car and then fired at least 33 shots with a 9mm handgun, police said. Five gunshot victims were in critical condition but all 10 wounded in the shooting were expected to survive.
At least a dozen others who escaped gunshot wounds were treated for smoke inhalation and other injuries.
One passenger, Jordan Javier, thought the first popping sound he heard was a book dropping. Then there was another pop. People started moving toward the front of the car, he said, and he realised there was smoke.
When the train pulled into the 36th Street station in the Sunset Park neighbourhood, people ran out and were directed to another train across the platform. Passengers wept and prayed as they rode away from the scene, Javier said.
"I'm just grateful to be alive," he said.
The station was open as usual Wednesday morning, less than 24 hours after the violence.
The subway system as a whole was operating normally on Wednesday, with police checking backpacks at some stations, including the Atlantic Avenue/Barclay's Center hub.
Tuesday's shooter fled in the chaos, leaving behind the gun, extended magazines, a hatchet, detonated and undetonated smoke grenades, a black garbage can, a rolling cart, petrol and the key to a rental van.
That key led investigators to James, who has addresses in Philadelphia and Wisconsin, NYPD Chief of Detectives James Essig said. The van was later found, unoccupied, near a station where investigators determined the gunman had entered the subway system, Essig said.
Rambling, profanity-filled YouTube videos apparently posted by James, who is Black, are replete with violent language and bigoted comments, some against other Black people.
In one video, posted a day before the attack, he criticises crime against Black people and says drastic action is needed.
Several videos mention New York's subways.
A February 20 video says the mayor and governor's plan to address homelessness and safety in the subway system "is doomed for failure" and refers to himself as a "victim" of the city's mental health programs. A January 25 video criticises Adams' plan to end gun violence.
Adams said in a video statement that the city "will not allow New Yorkers to be terrorised, even by a single individual."
