Washington ? Air France canceled several flights to the United States after U.S. officials, on heightened alert for holiday terror attacks, passed on “credible” security threats involving passengers planning to fly from Paris to Los Angeles, U.S. and European officials said Wednesday.

U.S. officials also ordered the emergency installation in California of dozens of sensors that sniff the air for biological pathogens that could be used in a terrorist attack.

Six flights scheduled for Wednesday and today were canceled — three headed to Los Angeles, and the three return flights to Paris. U.S. officials have repeatedly warned that al-Qaida terrorists may be eyeing Los Angeles International Airport.

U.S. officials were also in intense security talks with officials from several other countries on Wednesday, as intelligence concerns again intensified about possible plans by the al-Qaida terror network to use aircraft to attack American targets. One industry official, who asked not to be identified, said a Mexican airline, Aeromexico, was another focus of U.S. concern.

Under close watch

At home, U.S. security officials closely watched activity at airports, train stations and public buildings, while police randomly stopped cars near the U.S. Capitol as they nervously watched for Christmas Eve disruptions.

U.S. officials said intelligence gleaned from overseas electronic intercepts and other means pointed to California as the location of highest concern. U.S. officials expressed fears about the use of a range of devices that included biological or chemical weapons, and a radiological or “dirty bomb,” officials said.

Department of Homeland Security officials decided the threat was credible enough to order the immediate placement in California of scores of additional outdoor air-sampling sensors that are designed to warn officials of any release of deadly microbes into the atmosphere. Dozens of such “Bio-Watch” sensors have been in use in a number of California cities and dozens of others, including Washington and New York, since last March.

Military and civilian emergency teams that respond to possible terrorist plots involving weapons of mass destruction have been placed on heightened alert around the country since Sunday, when the government raised the national threat status to orange or “high risk.”

Police prepare to check an unattended bag at Roissy-Charles de Gaulle airport, north of Paris, after three Air France commercial flights between Paris and Los Angeles were cancelled due to security concerns. The U.S. Embassy in Paris asked the French government to cancel the flights Wednesday, as well as Air France flights scheduled on Christmas Day.

While U.S. officials have expressed their most pointed concerns about California, they remain worried about possible attacks in other cities, including Washington, New York and Las Vegas and numerous parts of the nation’s critical infrastructure.

“I’m gravely concerned,” said one government official whose days have been a blur of briefings in secure government installations since Saturday morning, the day before Homeland Security Secretary Tom Ridge placed the nation on orange alert. “I haven’t gotten a lot of sleep for days.”

One high-ranking government official who also has spent the past week being briefed on the government’s efforts, said that “lives are at stake here. I’m very worried about it.”

Suspicious passengers

A spokesman for French Prime Minister Jean-Pierre Raffarin said the decision to cancel the six Air France flights came early Wednesday after American authorities notified France that “two or three” suspicious people, possibly Tunisian nationals, were planning to board the flights.

The French Interior Ministry said the flights were canceled at the request of the U.S. Embassy in Paris. A spokesman for Raffarin said the United States had threatened to refuse the planes permission to land in Los Angeles if they took off.

But U.S. officials refused to confirm that they had made the request, leaving the question of who actually ordered the cancellations unclear.

The United States handed French authorities the names of suspicious people who may have intended to board the flights, but no people by those names went through airport security checks, and no arrests were made, the Interior Ministry said.

French television station LCI reported that American authorities believed members of al-Qaida may have been planning to board the planes. The Interior Ministry declined to comment on whether any al-Qaida members figured into the incident.

American officials said the U.S. government was comparing data it had compiled on passengers preparing to board flights entering the United States, as well as data on the flight crews on those flights, with terrorist watch lists it has compiled.

“We are looking at both passengers as well as flight crews,” the official said.

High-level talks

Officials from the U.S. Homeland Security Department, including Ridge, had been meeting with French officials in recent days over concerns about a possible terrorist attack.

In addition, Secretary of State Colin Powell conferred by telephone Wednesday afternoon with French Foreign Minister Dominique de Villepin about the security situation. Bush administration officials declined to provide details but suggested concerns extended beyond the Air France flights.

A Homeland Security official in Washington, speaking only on condition of anonymity, said the United States had been working with a number of governments overseas to help them increase their security measures at airports. The official said American officials had passed on to other governments a “very credible threat” of possible attacks originating overseas.

U.S. officials have been working to get foreign airlines to provide American officials with more passenger information on people aboard the flights that originate overseas and travel to the United States, said an official who spoke earlier this week on condition of anonymity. France and Mexico were of particular concern in this regard, the official said.

Despite the threats, U.S. officials were encouraging all U.S. travelers to keep their scheduled holiday plans, noting the “significantly enhanced security here and overseas” since the Sept. 11, 2001, attacks.

At Los Angeles International Airport, security had already been tightened to its highest level in two years. Los Angeles operates one of the busiest airports in the world and was the hoped-for target in a foiled al-Qaida bomb plot planned for around New Year’s Day 2000.