The man charged with a terrorism offence over the shooting of 10 people on the New York subway posted dozens of videos ranting about race, violence and his struggles with mental illness.

One was a silent shot of a packed New York City subway carriage in which he raises his finger to point out passengers, one by one.

As police arrested Frank R James, 62, on Wednesday over the Brooklyn shooting they were still searching for a motive from a flood of details about his life.

An erratic work history. Arrests for a string of mostly low-level crimes. A storage locker with more ammunition and hours of rambling profanity-laced videos on his YouTube channel that point to a deep, simmering anger.

"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," says James in a video where he calls himself Prophet of Doom.

After a 30-hour manhunt, James was arrested without incident after a tipster, thought by police to be James himself, said he could be found near a McDonald's on Manhattan's Lower East Side.

Mayor Eric Adams triumphantly proclaimed "We got him!"

Police said their top priority was getting the suspect, now charged with a federal terrorism offence, off the streets as they investigate their biggest unanswered question, why?

A prime trove of evidence, they said, is his YouTube videos. He seems to have opinions about nearly everything - racism in America, New York City's new mayor, the state of mental health services, 9/11, Russia's invasion of Ukraine, and black women.

A federal criminal complaint cited one in which James ranted about too many homeless people on the subway and put the blame on New York City's mayor.

"What are you doing, brother?" he said in the video posted on March 27. "Every car I went to was loaded with homeless people. It was so bad, I couldn't even stand."

James then railed about the treatment of black people in an April 6 video cited in the complaint, saying: "And so the message to me is: I should have gotten a gun, and just started shooting."

In a video posted a day before the attack, James criticises crime against black people and says things would only change if certain people were "stomped, kicked and tortured" out of their "comfort zone".

Surveillance cameras spotted James entering the subway system turnstiles on Tuesday, dressed as a maintenance or construction worker in a yellow hard hat and orange working jacket with reflective tape.

Police say fellow riders heard him say only "oops" as he set off one smoke grenade in a crowded carriage as it rolled into a station.

He then set off a second smoke grenade and started firing, police said.

In the smoke and chaos that ensued, police say James made his getaway by slipping into a train that pulled in across the platform and exited after the first stop.

Left behind at the scene was the gun, extended magazines, a hatchet, detonated and undetonated smoke grenades, a black rubbish bin, petrol and the key to a van, police said.

That key led investigators to James, and clues to a life of setbacks and anger as he bounced among factory and maintenance jobs and was sacked at least twice.

Investigators said James had 12 prior arrests from 1990 to 2007, including for possession of burglary tools, criminal sex act, trespassing, larceny and disorderly conduct.

James was not prohibited from purchasing or owning a firearm.

Police said the gun used in the attack was legally purchased at a pawn shop in 2011.

A search of James' storage unit and apartment turned up at least two types of ammunition, including the kind used with an AR-15 assault-style rifle and a taser.