The FBI has arrested a 24-year-old Colorado man on charges of making false statements to U.S. federal agents in an ongoing terrorism investigation, while supporting documents contend the man admitted receiving weapons and explosives training from al-Qaeda in Pakistan.

Najibullah Zazi was arrested late Saturday after undergoing three days of interrogations by the FBI. Zazi, a legal U.S. permanent resident from Afghanistan, was due to appear in federal court on Monday.

Also arrested were Zazi's father, Mohammed Wali Zazi, 53, in Denver, and an associate, Ahmad Wais Afzali, 37, of New York City, the U.S. Justice Department said Sunday.

Both also were charged with making false statements to federal agents, a charge that carries a maximum penalty of eight years in prison. Court appearances for both also were set for Monday.

Najibullah Zazi has repeatedly denied to reporters any connection to al-Qaeda or to a purported bomb plot.

A senior U.S. intelligence official in Washington said Friday that Zazi has indicated that he is directly linked with al-Qaeda. The official, who spoke on condition of anonymity in order to discuss intelligence matters, said Zazi played a crucial role in an intended attack but that it was not immediately clear what the targets were.

The FBI is investigating several individuals in the United States, Pakistan and elsewhere in its probe into an alleged plot to detonate explosive devices in the United States, the Justice Department said in a statement.

Possible targets unknown, official says

"The arrests carried out tonight are part of an ongoing and fast-paced investigation," said David Kris, assistant attorney general for national security. "It is important to note that we have no specific information regarding the timing, location or target of any planned attack."

A joint FBI-New York Police Department task force feared Zazi might have been involved in a potential plot involving hydrogen peroxide-based explosives, according to two other law enforcement officials, who spoke on condition of anonymity because they were not authorized to speak about the investigation.

In supporting documents filed with the court, investigators say Zazi admitted to FBI agents last week that in 2008 he received weapons and explosives training from al-Qaeda in Pakistan's Federally Administered Tribal Areas.

The terrorism probe gathered momentum after Zazi rented a car and drove from Denver to New York, crossing into Manhattan on Sept. 10.

Zazi said he went to New York to resolve some issues with a coffee cart he owns in Manhattan, then flew home to Denver. On Monday, FBI agents and police officers with search warrants seeking bomb materials searched three apartments and questioned residents in the New York neighbourhood where Zazi stayed.

The FBI also searched Zazi's rental car and laptop during the New York trip and listened in on telephone conversations, according to the affidavits. The searches turned up a laptop computer that contained an image of nine pages of handwritten notes, according to court documents filed with the arrest warrant.

Those notes included instructions about how to build explosives and detonators, the affidavits state.

Zazi was asked about the notes during FBI interviews last week and said he knew nothing about them, the documents said.

Zazi allegedly told federal agents that he must have unintentionally downloaded the notes along with a religious book. He said he deleted the book within a few days after realizing it discussed jihad, the affidavit said.

Email, phone conversations investigated

However, federal agents suspect Zazi received the notes via email.

Authorities have found images of the notes in two email accounts with similar passwords. One of the accounts has a nine-digit password that is identical to the password for an email account that Zazi told investigators this week was his, the affidavit said.

Authorities suspect Zazi controls both email accounts that received copies of the handwritten notes, according to the affidavit.

An arrest warrant affidavit says FBI agents listened in on a phone conversation around Sept. 11 in which Afzali, a U.S. legal permanent resident from Afghanistan, told Zazi that he had spoken with authorities.

Afzali is alleged to have lied to authorities about that conversation when federal agents asked him about it Thursday, according to the affidavit.

The Justice Department also says Zazi's father, Mohammed Zazi, a naturalized U.S. citizen who was interviewed last week by the FBI, lied when asked whether he knew anyone by the name of Afzali and said he didn't.

Zazi was born in Afghanistan in 1985, moved to Pakistan at age seven and emigrated to the United States in 1999.

He returned to Pakistan in 2007 and 2008 to visit his wife, according to his lawyer Arthur Folsom.