Officials identified the suspect as Andrew Arrabaca, a 43-year-old US Army veteran living in Harlem.
On Monday morning, a man appeared to pour petrol from a bucket onto the stairs of 26 Federal Plaza, an office tower that also houses the FBI's New York field office, then set it on fire, according to authorities and local news videos.
The suspect, who brought anti-Immigration and Customs Enforcement literature, also ignited fireworks, authorities said.
The Department of Homeland Security said on X that a Federal Protective Service officer who detained the man, described by DHS as "an anti-ICE rioter," sustained minor injuries.
Two other people were also lightly injured, James Barnacle, FBI assistant director in charge of the New York office, told a news conference.
Barnacle said the suspect was "an anti-American, anti-government extremist."
James McDonald, deputy US Attorney for the Southern District of New York, said an investigation was continuing and no decision has yet been made on charges.
Arrabaca could not be reached for comment, and it was unclear if he had a lawyer.
Arrabaca served in the Army from 2001 to 2005 and serviced Patriot missile systems, officials said.
Investigators traced the suspect to his home in Harlem, authorities said. Officials said he arrived in New York by subway on Sunday and spent the night on the street in Lower Manhattan.
"What occurred outside 26 Federal Plaza this morning was deeply disturbing. I'm relieved no one was seriously injured and that a suspect is in custody," New York City Mayor Zohran Mamdani told reporters.
