Three Pakistani men said to have supplied funds to Times Square car bomb suspect Faisal Shahzad were arrested in a series of raids as the FBI followed the money trail of the failed attack.
Investigators said it was not yet clear whether the three men knew how the money was going to be used.
The men - two seized in the Boston, Massachusetts, area and one in Maine - were arrested as authorities searched homes and businesses in co-ordinated raids in the Boston suburbs, on New York's Long Island and in New Jersey.
They were arrested for immigration offences - administrative, not criminal, charges and were not charged with any terror-related crimes. Their names were not released.
The raids followed evidence gathered in the investigation into the Times Square bomb attempt two weeks ago.
FBI spokeswoman Gail Marcinkiewicz said yesterday there was "no known immediate threat to the public or any active plot against the US".
In Washington, attorney general Eric Holder said investigators believed there was evidence that the men were providing Shahzad, a Pakistan-born US citizen, with money, but they have yet to determine whether the men knew the funds might have been intended for a terrorist act.
A top Massachusetts law enforcement official said investigators were not sure whether the two Boston-area men were willing accomplices or simply moving funds, as is common among people from the Middle East and central Asia who live in the US
"These people might be completely innocent and not know what they were providing money for," the official said, "but it's clear there's a connection."
Republican senator Susan Collins said there was "not a direct tie" between a man arrested in South Portland, Maine, and the car bomb suspect.
Authorities have been investigating whether Shahzad - who authorities say needed only a few thousand dollars to buy the used SUV and the bomb components used in the attempted May 1 attack - was financed from overseas.
A law enforcement official said money was passed to Shahzad through the informal transfer networks known as hawalas.
Muslim immigrants have for years used hawalas, which rely on wire transfers, couriers and overnight mail and are cheaper and quicker than banks, to send cash to their families overseas.
But since the September 11 2001 attacks, authorities have worked to dismantle the system, fearing it allows terrorists to raise and launder money.
Tracking the money to Shahzad through a hawala system would involve interviewing a large number of people and would probably be a more difficult task than tracing funds through more conventional financial networks, the official said.
Two of the men under arrest overstayed their visas and the third was already in removal proceedings, said another law enforcement official.
Shahzad, 30, has waived his right daily to appear in court since his arrest for allegedly trying to blow up a van packed with petrol and propane outside Times Square's busy restaurants and Broadway theatres, US attorney Preet Bharara said.
He is continuing to provide investigators with information.
"We are doing exactly what, I think, people want us to do, and that is to make sure we get all the information we can with respect to any associates he may have, and other information that would help us to prevent anything further from happening in the US," the prosecutor said.
Kifyat Ali, a cousin of Shahzad's father, has called Shahzad's detention "a conspiracy so the (Americans) can bomb more Pashtuns", a reference to a major ethnic group in Peshawar and the nearby tribal areas of Pakistan and south-west Afghanistan.
He has insisted that Shahzad "was never linked to any political or religious party" in Pakistan.
Shahzad, a budget analyst from Bridgeport, Connecticut, returned to the US in February from five months in Pakistan, where authorities say he claimed to have received training in making bombs.
Meanwhile in Washington, a senior military official said Pakistani authorities had arrested a man claiming to be an accomplice of Shahzad.
The official, who spoke anonymously, was unable to say what information the suspect may have provided.
* (Reuters) New York City police cleared several blocks near Union Square overnight to investigate a suspicious vehicle but reopened streets a few hours later when it was determined the car posed no danger, police said.
Local news reports said what appeared to be two gasoline canisters were seen in the back seat, leading police to call in the bomb squad to investigate.
But police gave the "all clear" signal after finding the owner of the car and determining there was no danger, a police spokesman said.
A police captain on the scene, who declined to give his name, said said there was no bomb, "just someone who did something they didn't know it was wrong."