The Denver shuttle bus driver at the center of a federal terrorism investigation has begun to cooperate with the authorities, admitting that he had perhaps unwittingly crossed paths in Pakistan with extremists allied with Al Qaeda, according to government officials briefed on the case.

The officials did not indicate whether the admissions of the man, Najibullah Zazi, made during three days of intense questioning by F.B.I. agents in Denver, clarified whether he had directly engaged in terrorist activity. And it is not known whether his account could mean he will be charged with any wrongdoing.

But Mr. Zazi’s discussions with the authorities did seem to shed light on the suspicions that led federal and New York counterterrorism officials to begin intensive inquiries and to raid apartments in a suburb of Denver and in Queens, New York, believing he was the central figure in some kind of nascent, if still murky, terrorist activity.

Mr. Zazi’s statements to government officials suggest a somewhat different narrative than the one he presented in interviews with reporters this week when he denied being affiliated with Al Qaeda and said he had no idea why the government would suspect him of wrongdoing. Indeed, there are now some indications that Mr. Zazi underwent training in explosives and bomb-making while overseas.