Homes in New York have been raided in relation to a counter-terrorism investigation, officials say.
Federal agents helped by local police began executing search warrants on residences in the borough of Queens on Sunday night.
Unnamed sources were quoted by US media as saying the raids were linked to an undercover surveillance operation.
Federal agents are reportedly due to brief senior members of Congress about the case.
New York Police Department spokesman Paul Browne confirmed to the Associated Press that searches were conducted by agents of a joint terrorism task force.
The New York Times quoted an anonymous senior law enforcement official as saying the authorities had not detected a specific plot, but were suspicious of the activities of a small group of people who "espoused a militant ideology aligned with al-Qaeda".
New York Senator Charles Schumer, who was briefed by FBI officials, said he could not discuss much of the case because many of the details were still classified.
"There was nothing imminent, and they are very good now at tracking potentially dangerous actions and this was preventive," he was quoted by the Associated Press as saying.
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