The US Department of Homeland Security said a man was in custody on Monday after lighting fireworks and pouring petrol outside a federal building in New York.
An FBI official said the man is a former US soldier, Andrew Arrabaca, a mechanic who worked on Patriot missile systems.
“He is an anti-American, anti-government extremist,” the FBI official told reporters.
The DHS described the suspect as an agitator and said a security officer tackled him as he lit fireworks and tried to cause more damage.
“Officers quickly recovered the individual’s bag, which contained what appeared to be two pellet guns and a manifesto,” the department said.
In a post on X, New York Mayor Zohran Mamdani said no one had been seriously injured in the attack and a federal investigation was under way.
“Our administration will continue to ensure that every New Yorker is safe in their city – and hold accountable any who threaten that,” Mr Mamdani said.
The incident occurred close to a building that holds offices for Immigration and Customs Enforcement, which has come under intense scrutiny for its hard-line tactics.
The building in lower Manhattan has been a popular location for protests and speeches against the Trump administration's immigration policies.
While the motive behind the incident is unclear, it comes after the US State Department last week sounded the alarm about the growing threat of “far-left terrorism”.
“Far-left political terrorism is resurgent, manifesting in violent terrorist acts across the Western Hemisphere, Europe, Asia and beyond,” the State Department said in a statement.
