More sensors to detect microbes ordered in Calif.

As flights between Paris and Los Angeles were being canceled Wednesday, U.S. security officials closely watched activity at U.S. 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 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.

With teams that would respond to use of weapons of mass destruction on standby alert, high-ranking U.S. government officials have held near round-the-clock meetings in recent days to sift through intelligence reports that suggest the possibility of terrorist activity in the United States this holiday week.

U.S. officials said intelligence gleaned from overseas electronic intercepts and other means points to California as the location of highest concern. U.S. officials expressed fears over 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 March.

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

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.

Elsewhere around the country:

Christmas vacations for many FBI and other federal agents were canceled.

Federal officials granted Chicago's request to temporarily bar planes from flying over downtown Chicago at altitudes below 3,000 feet without approval.

A Delta terminal at New York's LaGuardia Airport was evacuated late Wednesday after a passenger breached security.

Los Angeles International Airport banned passengers from being picked up or dropped off at the airport curb for at least two weeks.

- Information from the Los Angeles Times, Associated Press and New York Times was used in this report.

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