The source said 24-year-old Najibullah Zazi, who until now had protested that he had no connection to Al Qaeda, changed his story Friday.
Zazi reportedly told officials that he had received explosives training and may plead guilty as part of a deal to cooperate with the government.
A senior intelligence official in Washington told The Associated Press 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 terrorist attack but that it was not immediately clear what the targets were.
The official went on to say that the plot was being directed from outside the United States.
Zazi had submitted to two eight-hour interrogations, Wednesday and Thursday, at the FBI offices in Denver, and he was called back for further questioning Friday.
The FBI's Joint Terrorism Task Force went through Zazi's home, as well as the nearby residence of his aunt, Rabia Zazi. The searches are part of a terrorism investigation that fed fears of a possible subway bomb plot and led to several police raids Monday in New York City.