Editor's Note: The source who warned federal authorities about a potential new terror threat in the U.S., as reported Thursday by Fox News, was accused Friday of lying about that threat. Click here for the full update.

Senior U.S. officials are concerned over recent intelligence indicating that the Pakistani Taliban, which orchestrated the failed Times Square bombing, may have successfully placed another operative inside the United States to launch a second attack, sources tell Fox News. Authorities, however, know very little about the potential operative or any possible plot.

"[We] don't know who it is and don't know where it is," one source said. "We know the guy's here, but don't know anything about him."

Based on the intelligence, authorities believe the Pakistani Taliban, also known as Tehrik-e Taliban Pakistan, would have directed the individual to attempt another Times Square-style operation, but not necessarily in New York City.

A senior intelligence official said the threat stream's lack of specificity makes it nearly impossible for the counterterrorism community to defend against such an attack. Any possible threat, however, does not seem to be imminent, with a senior counterterrorism official saying he was "unaware" of any "imminent threats" against the U.S. homeland.

Nevertheless, the Pakistani Taliban has been looking to make up for its previous failure. Authorities believe the subject of the latest intelligence would use "a similar mechanism" and the "same modus operandi" employed by 31-year-old Faisal Shahzad in May, mostly "because it's easily accessible here," as one source put it.

In the months leading up to his attack, Shahzad purchased fertilizer, propane gas, fireworks and other components from stores in Connecticut and Pennsylvania. But the bomb he ultimately built and packed inside a sport utility vehicle did not detonate properly.

If someone successfully set off such a bomb, the effects would be "devastating," according to federal prosecutors. In June, FBI agents built and tested a device identical to Shahzad's, except this time they made sure the bomb actually detonated.

"Had the bombing played out as Shahzad had so carefully planned, the lives of numerous residents and visitors of the city would have been lost and countless others would have been forever traumatized," federal prosecutors said in court documents filed in the Shahzad case two weeks ago. "This is to say nothing of the significant economic and emotional impact a successful attack would have had on the entire nation."

Authorities are describing the latest threat as "credible but not specific," and they are "very nervous," according to the sources. It's unclear exactly when or how the intelligence was obtained, but one source said it was "corroborated" by authorities. Others were unable to say the intelligence had been corroborated.

"In many cases, intelligence we get ends up washing out," said the senior counterterrorism official, who would not specifically discuss or even confirm the latest intelligence.

It's also unclear when or how the operative would have entered the United States, but the recent intelligence says he would have been sent from Pakistan's tribal areas, where only months earlier associates of the Pakistani Taliban trained Shahzad to build and detonate bombs, according to the senior intelligence official.

After living in the United States for a decade and becoming a naturalized U.S. citizen, Shahzad left for Pakistan in late 2009. He spent five months there before returning to Connecticut to prepare his attack.

As for the subject of the latest intelligence, he could be anywhere in the United States, and officials are not convinced he would necessarily target New York City.

"It's not surprising this day and age that an individual is residing in or traveled to the United States in hopes of pulling off some sort of attack," the senior counterterrorism official, speaking generally, said. "We are 'Target Number One' for terrorists, and it requires a constant vigilance."

In particular, federal officials have become increasingly concerned about U.S. citizens who, like Shahzad, "choose to serve as an operative for a foreign terrorist organization," as federal prosecutors put it.

In court documents filed in the Shahzad case, prosecutors said that "under the cover of their U.S. citizenship" such individuals can "travel freely around the world" and "can remain in the United States undetected."

In a video released by the Pakistani Taliban two months after the failed Times Square attack, Shahzad said it is "not difficult at all to wage an attack on the West, and specifically in the U.S."

"Get up and learn from me and make an effort," he said in the video, recorded eight months before its release. "Nothing is impossible if you just keep in mind that Allah is with you."

Still, senior U.S. officials said recently that even failed attacks like the Times Square plot can ultimately be successful in some ways.

"These smaller attacks -- even if unsuccessful -- may still generate significant publicity and therefore might have both a psychological and an economic impact," FBI Director Mueller said last week during an intelligence-reform conference organized by the Bipartisan Policy Center in Washington.

Two weeks earlier, the head of the National Counterterrorism Center, Michael Leiter, told a Senate hearing that "additional attempts by Al Qaeda affiliates ... to attack the U.S., particularly attempts in the homeland, could attract the attention of even more Western recruits, thereby increasing those groups' threat to the homeland." And despite some setbacks for the Pakistani Taliban, he said, the group has "time and time again proven its resilience and remains a very capable and determined enemy."

The recent intelligence regarding the Pakistani Taliban has no connection to the U.S. State Department's recent alert urging Americans to use caution when traveling to Europe. U.S. officials have emphasized that the intelligence leading to the travel alert did not indicate a direct threat to the U.S. homeland, but they have been careful not to rule out other threats or to address other threat information.

In fact, the senior intelligence official said there are five major threat streams -- three aimed at Europe and two aimed at the United States -- that U.S. authorities are following right now.

Asked about "the current threat environment here in the United States" during a press conference in Washington last week, Attorney General Eric Holder insisted the "threat screening that precipitated the [travel] alert is all directed at Europe."

"That does not mean, however, that we're letting our guard down with regard to the United States," he added. "We have certainly seen over the past year attempts by Al Qaeda or its affiliates to attack the United States. We saw that in Detroit [with the failed Christmas Day bombing]. We saw that in Times Square. And so we are mindful of the fact that the threat to our homeland is a continuing one."

In September, the State Department designated the Pakistani Taliban a foreign terrorist organization, saying the group "draws ideological guidance" from Al Qaeda and is "attempting to extend their bloody reach into the American homeland." Their primary goals are to topple the Pakistani government, force Pakistani troops out of areas along the Pakistan-Afghanistan border, and to establish Islamic law in the region, according to U.S. officials.

In recent years, the Pakistani Taliban has carried out several attacks against U.S. interests overseas, including a deadly attack on a CIA base in Afghanistan, but the Times Square attempt was the group's first attack outside South Asia.

Last week, after pleading guilty to 10 terrorism-related counts, Shahzad was sentenced to life in prison for his role in the attempted bombing.

An FBI spokesman said he could not offer any information for this article, and spokesmen for the Department of Justice declined to comment.