Holder told a Senate Appropriations subcommittee Shahzad is cooperating with investigators and the information he has given has been useful, The Washington Post reported.
Shahzad, 30, a naturalized U.S. citizen born in Pakistan, is suspected of leaving a sport-utility vehicle loaded with explosives in New York's Time Square Saturday night. The explosives failed to detonate.
"As we've seen in prior investigations, the giving of Miranda warnings has not deterred people from talking to us, and Mr. Shahzad is continuing to cooperate with us," Holder said.
"We will continue to pursue a number of leads as we gather intelligence relating to this attempted attack."
Holder rejected Republican criticism of the administration's handling of the would be car bombing in Times Square, saying the Supreme Court has ruled every American has a right to hear his Miranda rights.
Investigators are examining the possible role of the Pakistani Taliban in the attempt.
Shahzad, arrested Monday just before his Emirates Airlines flight to Dubai was to take off from New York, Wednesday waived his right to a speedy arraignment, which The New York Times said was an indication of his cooperation with authorities.
A senior official in the administration told the Times there are "no smoking guns yet" about the Taliban's involvement, but others spoke of strong indications Shahzad knew some Taliban members who may have helped train him.
Pakistani media reports have said Shahzad is the son of a retired air vice marshal and he and his wife have two children.
U.S. officials told the Times Shahzad's interrogation has provided evidence the Pakistani Taliban trained him months prior to Saturday's attempted attack.
The officials say Shahzad also reportedly discussed his contacts with the Taliban, a group that this week sought to show through videos its leader Hakimullah Mehsud was not killed as originally believed in a U.S. drone strike in January. Mehsud's group claimed responsibility for the Times Square incident.
One question that remains unanswered is the source of the cash Shahzad used to buy the SUV used in the attempted bombing and the ticket for his flight to Dubai.
"Somebody's financially sponsoring him, and that's the link we're pursuing," one official told the Times. "And that would take you on the logic train back to Pak-Taliban authorizations."