The U.S. government secretly dispatched scores of casually dressed nuclear experts with hidden radiation equipment to scour five major American cities for "dirty bombs" during the year-end holidays, Fox News has confirmed.

Experts were sent to Washington, D.C., New York, Las Vegas, Los Angeles and Baltimore to make sure no radiological devices had been planted. The Department of Homeland Security (search) said the information that led to the move was specific to concerns about the holidays.

The experts were first sent out around Dec. 21, the day that the nation's terror alert level was raised from yellow, or "elevated," to orange, or "high."

"It really shouldn't come as a surprise that the Department of Homeland Security put security measures in place," one official told Fox News.

The story was first published in Wednesday's edition of the Washington Post.

dirty bomb (search) could kill or injure people by exposing them to radioactive materials, terrorism experts say. U.S. officials believe that the Al Qaeda has been sparing no effort trying to use one, sources said.

A dirty bomb is a conventional bomb, packaged with radioactive material, that when detonated would spew radiation across a small area. Experts say a dirty bomb containing one kilogram of plutonium exploding in the center of Munich, Germany, for example, could cause 120 cancer cases.

"I think a dirty bomb or a radiological device is probably the most likely weapon a terrorist group will use ... it builds off what terrorists already know how to use — a truck bomb or a car bomb," Joseph Cirincione, director of the Non-Proliferation Project in Washington, D.C., told Fox News. It's good at "spreading terror and chaos throughout the region," he added.

Although only people in the immediate vicinity would actually be harmed by the bomb itself, radiological particles could cause long-term health problems like cancer.

"But the terror — that would spread," Cirincione said. "It seems almost like an ideal terror weapon and I, and several others, are surprised we haven't seen one yet."

Hundreds of nuclear and bioweapons scientists remain on high alert at several military bases around the country, ready to fly to any trouble spot, sources confirmed to Fox News.

"The threat for a dirty bomb remains at all times," Michael White, a former New York Police Department bomb squad commander, told Fox News. "All the people of authority are concerns about this particular threat any time of the year. ... It definitely made sense to add any precaution you could."

The chances of a dirty bomb being used in the U.S., instead of a more conventional terrorist weapon, were small, White added, but "it would be [incredible] if the military and other authorities failed to prepare for that possibility."

Some lawmakers said the threat is a reason to work with Russia and other countries to account for missing radioactive material.

"Certainly we know Al Qaeda had an interest in a radiological device," Rep. Adam Schiff, D-Calif., told Fox News. "We're not taking, I think, the full steps that we ought to guard against this threat."

The United States needs to strengthen export controls of radiological material, he added.

Rep. Mark Foley, R-Fla., countered that Bush administration's recent success with getting Libya to dismantle its nuclear weapons program and cut terrorist ties is proof that it takes the nuclear threat seriously.

"The deployment of scientists [during the holidays], I think, was a forward-thinking opportunity by this administration to see if we've had any infiltration in this country," Foley said. He added that officials needed to determine who had — and was willing to use — dangerous materials against humanity.

In other news, more details emerged about why Air France flights from Paris to the United States were canceled over the holidays.

French authorities confirmed that U.S. officials had asked them to track down an Afghan who was thought of by the United States as a terrorist who might be planning an attack using an airplane flying from Paris to Los Angeles.

French judicial sources said the man, identified as Abdou Hai, was believed to be a passenger who didn't turn up for a scheduled Air France flight on Christmas Eve. The flight was one of six between Paris and Los Angeles that authorities eventually canceled following security talks between the United States and France.

Terror Alert Downgrade Possible

The United States may soon be ready to downgrade the national terror alert level, officials told Fox News.

"The level and volume of very credible threat reporting has gradually dropped off," a Homeland Security official said on Tuesday.

The official said the reduction in credible threat reporting began last weekend, and that if it continued, key decision-makers would consider lowering the threat level.

Returning to "elevated" terror alert status would mean a ratcheting-down of protective measures around the country, the official told Fox News.

He added that the current volume of overall threat information received by authorities had not declined, but explained that this was to be expected with more security personnel actively engaged and citizens more inclined to report information they believe may be significant.

Homeland Security officials had announced on Monday that the country would stay in a high state of terror alert due to intelligence suggesting attacks might still be planned.

The White House said DHS continues to take action every day to protect the nation from potential terrorism at home.

"We do live in a post-Sept. 11 world, and it's important that we continue to do everything we can to protect the American people," said White House spokesman Scott McClellan. "And that's exactly what we have been doing."

Regarding any of the threats to aviation, one Homeland Security official told Fox News "we continue working with our domestic and international aviation partners providing them the latest information we have."

Up until early this week, officials said human intelligence, computerized watch lists and intercepted communications continued to suggest attacks may be planned.

Some wondered whether the surfacing of yet another Usama bin Laden audiotape may be a prelude to an attack somewhere in the world. Intelligence officials confirmed Monday that the voice on the latest tape was most likely bin Laden's.

"I think the question regarding that is still up in the air, but whether we should be on a heightened state of alert, I think that was the proper move," said former CIA operations officer Mike Baker.

Fox News' Mike Emanuel, Liza Porteus, Anna Stolley and Kelly Wright contributed to this report.