Three men of Afghan origin, including one suspected of training at an Al-Qaeda camp in Pakistan, are due to appear in court on Monday charged with lying to authorities investigating a plot to launch attacks in the United States.
The men, all legal US residents, were arrested at the weekend after days of speculation following a series of FBI raids in the western US state of Colorado and the Queens borough of New York City.
Najibullah Zazi, 24, and his 53-year-old father Mohammed, arrested in Colorado late on Saturday, will appear in a Denver federal court, while Ahmad Wais Afzali, 37, will stand before a court in New York where he was taken into custody.
"Each of the defendants has been charged by criminal complaint with knowingly and willfully making false statements to the FBI in a matter involving international and domestic terrorism," the US Justice Department said.
Authorities reportedly detained the men as they consider making more serious charges against them.
Officials cited by the New York Times said the arrests were quickly made as the attack plot could have been further advanced than they knew, and counterterrorism agencies were preparing for world leaders descending on New York this week for the United Nations General Assembly.
The FBI is also investigating other individuals "in the United States, Pakistan and elsewhere, relating to a plot to detonate improvised explosive devices in the United States," according to affidavits filed to support the arrests.
New York City police commissioner Raymond Kelly also suggested more arrests are possible.
"I think it's important to note that in many ways, this investigation is only just beginning. It has many different avenues to take," he said, adding that leads in the case are being explored "in New York, in Denver, and in other locations in the country as well."
The arrests come after three days of voluntary questioning of Najibullah Zazi in Colorado.
David Kris, assistant attorney general for national security, described the arrests as "part of an ongoing and fast-paced investigation."
"It is important to note that we have no specific information regarding the timing, location or target of any planned attack," he added.
The CNN network, citing sources close to the investigation, reported the target was a major New York transportation hub, such as a rail or subway station.
Najibullah Zazi, a bus driver in Colorado, possessed a video of New York's Grand Central Station, the network said.
US media also said authorities found 14 new black backpacks in the New York raids that fueled concern the men may have been planning to use them to carry suicide bombs.
Stadium security officials said the Federal Bureau of Investigation had sent them alerts about a possible threat.
Justice Department officials said Sunday they had intercepted a number of phone calls between Najibullah and Mohammed Zazi and Afzali where the defendants discussed Afzali being interviewed by authorities.
Najibullah Zazi told Afzali his car had been stolen and that he feared he was being "watched," according to the affidavits.
Afzali, a long-time imam in Queens, then allegedly asked him whether there was any "evidence in his car," to which Najibullah replied no.
In a search of Najibullah Zazi's rental car in New York -- where he had been visiting Afzali -- officials found a digital image of handwritten notes "regarding the manufacture and handling of initiating explosives, main explosives charges, explosives detonators and components of a fusing system," according to the affidavit.
When shown the notes, Najibullah Zazi "falsely asserted that he had never seen the document before," officials said.
In interviews with FBI agents in Denver, Najibullah Zazi is said to have admitted that on a 2008 trip to Pakistan he "attended courses and received instruction on weapons and explosives at an Al-Qaeda training facility."
But in a telephone interview with Zazi, reported in the Denver Post newspaper on Saturday, he denied admitting any link to Al-Qaeda or involvement in an attack plot.
Afzali was an informant who had previously alerted authorities to criminal activity.
"He had always been helpful and candid and useful to them," his lawyer said, quoted Sunday in the Times. Permanent US residents Najibullah Zazi and Afzalia, and naturalized US citizen Mohammed Zazi, face eight years in prison if convicted of the false statements charges.