The United States charged for the first time Sunday that the Pakistani Taliban was behind a Pakistani-American's failed attempt to detonate a car bomb in the heart of New York City.
"We've now developed evidence that shows that the Pakistani Taliban was behind the attack," Attorney General Eric Holder said on ABC television's current affairs talk show "This Week."
"We know that they helped facilitate it. We know that they probably helped finance it, and that he was working at their direction."
Faisal Shahzad, the 30-year-old son of a Pakistani air force officer, was pulled off a plane to Dubai and arrested Monday for allegedly leaving a sport utility vehicle rigged to explode in New York's Times Square on May 1.
The United States has responded by stepping up pressure on Pakistan to crack down on Islamic extremists operating in safe havens in tribal areas along Pakistan's rugged border with Afghanistan.
The New York Times said General Stanley McChrystal, the US commander in Afghanistan, urged Pakistan's General Ashfaq Parvez Kayani in Islamabad on Friday to quickly begin a military offensive in North Waziristan, the stronghold of Al-Qaeda and the Pakistani Taliban.
And Secretary of State Hillary Clinton starkly warned of "very severe consequences" if a terror attack against the United States were traced back to Pakistan.
"We've made it very clear that if -- heaven-forbid -- an attack like this that we can trace back to Pakistan were to have been successful, there would be very severe consequences," Clinton told CBS's "60 Minutes" program to air later Sunday.
Clinton said there had been a "sea change" in cooperation by Pakistani authorities but she added, "We want more."
John Brennan, the White House deputy national security adviser, charged that Shahzad was trained and funded by the Tehreek-e-Taliban Pakistan, or TTP.
"He had extensive interaction with the TTP. And this is something that we are, again, looking at very carefully, understanding the extent of that interaction and the extent of the direction and guidance that was given to him," he told Fox News Sunday.
Brennan portrayed Shahzad as having been influenced by the "murderous rhetoric" of Al-Qaeda and the Pakistani Taliban, but told CNN's 'State of the Union" show that he had been "very cooperative" under questioning.
He said Shahzad had traveled back and forth six times to Pakistan over the past decade, working with the TTP over a period of several months on his latest trip, which stretched from mid-2009 to his return to the United States in February.
"What we are trying to do is determine now exactly who helped him, who worked with him and making sure we are able to uncover and then to address successfully these individuals who are trying to carry out other attacks," Brennan said.
He told CBS's "Face the Nation" that individuals in Pakistan "have been identified as being complicit in this."
"Again, we're working with the Pakistanis right now," he said. "We're trying to stay ahead of this curve. We've been able to find out things that we didn't know in the immediate aftermath of this attempted attack.
The Pakistani Taliban claimed responsibility for the failed attack a week ago, but it was initially discounted because the plot was so sloppily executed and the bomb so primitive it appeared to be the work of an amateur acting alone.
But US authorities have since concluded that the Shahzad case reflects a change in tactics by Islamic extremists, who have been hammered over the past year by a deadly onslaught of US missile strikes.
When coupled with Nigerian Umar Farouk Abdulmutallab's alleged attempt to blow up a US-bound airliner on Christmas Day with explosives sewn in his underwear, the New York bombing signaled a shift to the use of hastily trained individuals to carry out attacks.
"Abdulmutallab was a singleton on that plane, but he obviously had the links back to Al-Qaeda in the Arabian Peninsula. Shahzad in Times Square had links back. But he drove that vehicle to Times Square alone," Brennan said on Fox.
He told CNN other people with roots in the south Asian region had been noted traveling back and forth.
"It shows they are trying to take advantage of individuals who may have been able to come to the United States and in some instances acquire United States citizenship. We have to remain on guard against that."
US lawmakers unveiled legislation Thursday to strip naturalized Americans of their citizenship if they are thought to have joined extremist groups like Al-Qaeda.