Crews of journalists clustered outside Afzali's home in Queens. No one answered the doorbell at the three-story brick residence, adorned with brightly colored flower boxes.

Carl Tobias, a law professor at the University of Richmond who has been tracking terrorist investigations around the country, said authorities could have made the arrests now because they feared too much information was getting out to the suspects. Additional charges could be filed later, he said.

The investigation escalated after Zazi rented a car and drove from Denver to New York, crossing into Manhattan on September 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. The FBI searched Zazi's rental car and laptop during the New York trip and listened in on telephone conversations, according to the affidavits.

On Monday, FBI agents and police officers with search warrants seeking bomb materials searched three apartments and questioned residents in the Queens neighborhood where Zazi stayed.

An arrest warrant affidavit says FBI agents intercepted a phone conversation around September 11 in which Afzali, a legal permanent resident from Afghanistan, told Zazi that he had spoken with authorities. "I was exposed to something yesterday from the authorities. And they came to ask me about your characters (sic). They asked me about you guys," Afzali told Zazi, according to the affidavit.

However, Afzali allegedly lied to authorities about that conversation when federal agents asked him about it Thursday, according to the affidavit.

The department says Mohammed Zazi, a naturalized US citizen who was interviewed last week by the FBI, lied when asked if he knew anyone by the name of Afzali and said he didn't. The FBI said it had wiretapped a conversation between Mohammed Zazi and Afzali during Najibullah Zazi's visit to New York.

The FBI also searched Zazi's apartment and his uncle and aunt's home last week in suburban Denver. Authorities have not said what they found.

Zazi was born in Afghanistan in 1985, moved to Pakistan at age 7 and emigrated to the United States in 1999. He returned to Pakistan in 2007 and 2008 to visit his wife, according to Folsom.