Passengers on half a dozen Air France (search) jets scheduled to fly Christmas eve and Christmas day between Paris and Los Angeles won't get to their destinations after officials grounded the planes Wednesday as part of its stepped-up security measures.

U.S. officials said they had "received credible threat information that was passed on to international partners" about passengers scheduled on Air France flights whose names matched those on terror watch lists.

Another senior U.S. official said the intelligence came from a variety of sources and was specific and precise enough to warrant cancellation of the flights.

U.S. officials declined comment on camera, but one expert told Fox News that a jumbo jet bound to Los Angeles from Europe would be a formidable weapon, threatening most of the country.

"They are going to fly over almost the entire United States getting to Los Angeles. It doesn't mean that L.A. was the ultimate target," said Heritage Foundation (search) senior fellow Peter Brookes. "If they had a flight crew that was an Al Qaeda operative, they could have changed course and gone into New York, into Washington, into St. Louis, into San Francisco. We don't really know at this point and we may never know since the flights never got off the ground."

Cancellation of the flights followed the decision by the Bush administration on Sunday to raise its terror alert status to "code orange," meaning chance of a terror attack had increased to "high." Among the scenarios that concerned officials was the possibility that a foreign airplane could be used as a missile, similar to the Sept. 11, 2001, terror attacks.

Analysts say it should be no surprise that the use of airplanes topped the list of possible threats to security. Khalid Sheik Mohammed, a former top gun with Al Qaeda now in U.S. custody, had stated as far back as 1989 that he wanted to fly a plane into the CIA.  In 2001, would-be shoe-bomber Richard Reid hoped to use his sneakers to explode an airplane headed from Paris to Miami.

Brookes said raising the threat level to code orange sent a signal for U.S. government operatives to go "into overdrive, especially on the intelligence side" by doubling efforts to look at possible terrorism scenarios.

Since the level was raised, Homeland Security Secretary Tom Ridge, Transportation Secretary Norman Minetta and Secretary of State Colin Powell (search) have briefed their counterparts in several countries.

Mexican flights were said to have drawn particular scrutiny, and though none were cancelled, officials said privately that other flights could be grounded later in the holiday period.

After cancellation of the flights, the Homeland Security Department (search) released a statement saying that it was working in conjunction with a number of foreign governments "to make sure they elevate their aviation security measures" and "enforce all existing security guidelines to thoroughly screen passengers and secure all areas of the airport."

The tightened security has taken its toll on holiday travelers, who were forced into slower and longer lines in order to face more rigorous security checks at airports.

"They have a security checkpoint just past where the 45-minute waiting area is, so we had about a 10-15 minute delay at that point," said one Denver area traveler.

At Los Angeles International Airport (search), the long wait often followed a long walk. Security officials there banned private cars from approaching the terminal to pick up or drop off passengers.

"We just felt that in looking at all the information given, that this was a further security enhancement we can add as a precaution," said Los Angeles Mayor James Hahn late Tuesday. Hahn added that he has been in constant contact with local, state and federal law enforcement partners.

Anti-aircraft missile batteries stood watch over Washington, D.C., and police randomly stopped cars on Capitol Hill.

In New York, extra security was apparent from Times Square to the malls. Mayor Michael Bloomberg said local police officers have canceled their holiday plans so that most Americans don't have to.

"This is what the security forces are supposed to do; so the rest of us can go spend time with our families and enjoy life," Bloomberg said.

But even with the stepped up security measures, Washington, D.C. House Delegate Eleanor Holmes Norton said that the Bush administration needs to do more to help localities and individuals deal with the increased security threat.

"What we do in orange alerts is extract people from crime in the streets and send them out to guarding monuments, securing places that are the responsibility of the United States. That's not fair," Norton said. "And you're going to hear howling and screaming now that we'll have an orange alert until after New Year's with all of the resources of the localities pulled into guarding the U.S. and away from guarding local neighborhoods and communities."

Norton added that people across the nation are not equally informed, and residents in small towns do not understand that they should be taking the same precautions as people in big cities.

Some people in urban areas said the code orange alert may have made them more cautious, but it also made them more uncomfortable.

"I guess it is nice just to have the warning, although you are not sure what to do. I think I am going to avoid Times Square on the New Year's," said one female shopper in New York City.

"I don't think it has any impact; any positive impact on the public; it doesn't give them any tools really to respond and it just creates unnecessary anxiety," said another New Yorker. 

Still, others said they were happy with the coordinated and extensive security efforts.

"There are cops all over the place there are patrol cars with their lights flashing all over the place. You've got to be more comfortable," said one man.

Fox News' Wendell Goler and Sharon Kehnemui contributed to this report.