Law enforcement officials now believe some of the information that led to upgrading the terrorist alert status to the second highest level last week was likely fabricated, a senior government official said Friday.

Authorities drew that conclusion based on polygraphs given to terrorist suspects interviewed by the government, said the official, speaking on condition of anonymity. The apparent fabrication was first reported by ABC News.

But that information was not the sole basis for the decision to raise the alert status, the official said, adding that the change from code yellow to orange was based on multiple intelligence sources and not a single tip.

Fox News has learned from other officials that America's greatest concern continues to revolve around plans for a biological or chemical attack, followed by a more "conventional" attack, which would then be followed by a radiological event.

ABC News quoted senior law enforcement officials in Washington and New York who said a captured member of Al Qaeda apparently made up a claim that New York, Washington or Florida was going to be hit with a "dirty bomb" this week. A dirty bomb is a conventional bomb laced with radioactive material.

The sources said the informant claimed a terror cell in Virginia or Detroit had developed a way to get a bomb past airport scanners in shoes, suitcases or laptop computers.

They said the suspect was given a lie-detector test after the Code Orange alert was issued last week. An ABC consultant said he flunked it.

But law enforcement officials told Fox News Friday that the ABC report is "exaggerated at best," and they disputed the report that the detainee outlined an elaborate plot involving a Virginia cell and a strategy to smuggle a dirty bomb in a laptop to attack those three regions.

Officials said a Guantanamo Bay detainee described a dirty bomb plot at the end of the Hajj, which concluded Thursday. The detainee's information was treated as a small part of the aggregate, and was not the sole cause of raising the nation's alert status.

Since the threat level was raised last week, the federal government has given lie-detector tests to suspected terrorists who provided some of the original intelligence about future attacks.

Authorities also are keeping tabs on roughly 600 Al Qaeda sympathizers in this country, said the official, who cautioned that there may be many more who are unknown to law enforcement authorities

The FBI is tracking about 1,000 people across the country who have been known to voice sympathy for Al Qaeda or other radical Islamic groups. Most of these people live in the 30 largest U.S. cities, with high concentrations in New York and Detroit.

Counterterrorism experts believe they have identified the people with the strongest Al Qaeda ties, including those who may be in contact with leaders of the terror network. Those people are under surveillance 24 hours a day, seven days a week. Officials say Al Qaeda-trained individuals are far greater in number in Europe and Asia, and their arrival in the United States is a top concern.

The great bulk of the intelligence comes from U.S. sources and methods, but within the last year, there has been a growing amount of information from foreign intelligence services.

Fox News has learned that the majority of intelligence behind the current Code Orange alert status came from signals intercepts, including e-mail or other Web-based communications.

There was a drop off in such communication last week, which caused some analysts to fear that an attack plan had "gone operational," and the attackers were maintaining radio silence.

After the alert was announced, some analysts hinted to Fox that there were as many as five to seven different plots around the world. That number has now been downsized to two or three serious threats targeting the U.S., with specific information pointing to Washington, D.C., and New York City, as well as Europe and Saudi Arabia.

Some intercepts have raised concerns about Khalid Sheik Mohammed, one of the suspected planners of the Sept. 11 terrorist attacks, since the information recently gained has involved his known associates and suggested plots consistent with his modus operandi.

Officials will not say, however, if his voice, name or aliases have been intercepted.

Elsewhere in the world, U.S officials told Fox that cooperating witnesses have provided a steady stream of intelligence and much of it has been "actionable…we've rolled up a few cells based on what we have gotten."

There is new information suggesting that Al Qaeda has stepped up efforts in the last six months for a dirty bomb, but analysts continue to believe it would be the most difficult attack for Al Qaeda to mount. It was known long before Sept. 11, 2001, that Al Qaeda had chemical weapons.

It is known that Al Qaeda has sought biological and radiological weapons but their degree of success in obtaining them is unclear.

Top counterterrorism officials loathe any attempt to quantify the number of people in the United States with ties to Al Qaeda, since the number fluctuates literally every day. Although analysts believe there may be several hundred Al Qaeda trained individuals in America, they are only comfortable saying they have information on about 60.

But sources say 10 to 20 of those 60 have maintained periodic contacts with suspected Al Qaeda operatives around the world.

Analysts also believe "cells are out there" in the United States, but they are divided as to the number or whether any are "operational" noting that U.S disruption efforts are never public, particularly when they are successful.

Meanwhile, in the nation's capital, signs of preparation for possible terrorism are everywhere. Members of Congress are being told to have necessary supplies ready in the event of evacuation while the government warns key industries about potential attacks.

"Everyone in (the Capitol) has remained calm but cautious," said Rep. Bob Ney, R-Ohio, chairman of the House Administration Committee. "There is not a panic situation here."

Fox News' Molly Henneberg and Carl Cameron and The Associated Press contributed to this report.