Three men have been arrested in connection with an alleged plot to launch an attack in the United States, the US Justice Department says.
Two Afghan-born men, a father and son, were arrested in Denver, Colorado.
A third man, also from Afghanistan, was later detained in New York, the department said.
The men are accused of making false statements related to "a matter involving international and domestic terrorism", the statement said.
The FBI was investigating several people "in the United States, Pakistan and elsewhere, relating to a plot to detonate improvised explosive devices in the United States", the Justice Department said in court documents related to the arrests.
US media have reported that the investigation was focusing on a possible plan to attack a public area in New York.
David Kris, assistant attorney general for national security, said the arrests were 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," AFP news agency quoted him as saying.
The arrests followed three days of questioning by federal authorities of Najibullah Zazi, the Colorado-based son.
Officials questioned Mr Zazi for three days prior to his arrest
Earlier this week officials searched the Denver home of Mr Zazi, a 24-year-old airport shuttle driver.
He and his father, Mohammed Zazi, 53, will appear in court in Colorado on Monday. Ahmad Wais Afzali, 37, will appear in court in New York the same day.
In a telephone interview with the Denver Post newspaper on Saturday, Najibullah Zazi denied media reports that he had admitted any link to al-Qaeda or involvement in terrorism.
"It's not true," Mr Zazi said. "I have nothing to hide. It's all media publications reporting whatever they want. They have been reporting all this nonsense."
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