A man has been arrested in New Jersey in connection with the attacks on the World Trade Centre, the second confirmed arrest in the investigation, police have said.
Jersey City police director James Carter said the unidentified man was taken into custody in an apartment in the city, which is across the Hudson River from New York.
"A joint effort between federal agents and Jersey City police resulted in the arrest of one man," Mr Carter said.
He declined to give the man's name, his age or his nationality. "It's an ongoing investigation," he said.
Carter would not specify the charges, but a police source said the arrest was in relation to Tuesday's attacks by two hijacked commercial aircraft on the 110-story twin towers of the World Trade Centre.
Earlier in Washington, law enforcement officials said 25 people who have been or will be questioned by the FBI are in custody for possible immigration violations.
In addition to the 25, the FBI on Friday arrested an unidentified person in New York described as a Middle Eastern man who had a fake pilot's licence.
Officials said he was a "material witness" in the probe and the FBI was seeking another unidentified person on a similar warrant, but it was not known whether this was the man arrested in Jersey City.
Such arrests as a material witness typically occur when the person has information highly relevant to a criminal probe and when there is a flight risk, the officials said.