DENVER - Across the country, more airline passengers could find themselves standing in line or sitting on planes delayed at the gate when a federal law requiring the screening of all checked baggage for bombs takes effect Friday.

The law requires airlines to use any of four methods: hand searches, bomb detection machines, bomb-sniffing dogs or the matching of every piece of luggage to a passenger on board a plane.

Currently, less than 10 percent of the 1.4 billion bags flown in domestic airliners' holds annually are screened for explosives by such methods.

For security reasons, airline officials declined to comment on how they plan to comply on Friday. But airport officials around the country said most airlines apparently will use bag-matching.

The technique is designed to prevent someone from checking a bag with a bomb and never boarding the aircraft. The approach already is used on international flights.

The precaution means that if a passenger fails to board a plane or gets off just before takeoff, airline workers will have to climb into the hold to remove his or her luggage. That could create delays in pulling away from the gate.

The measure would also fail to stop a suicide bomber. In addition, plans call for requiring the bag match to be done when a passenger first boards a plane and not done a second time for a connecting flight, said a government source, speaking on condition of anonymity.

Passenger advocate David Stempler, president of the Air Travelers Association, said that concession would mean only an incremental improvement in security. But he said it could go a long way toward reducing the possibility of long check-in lines.

"Direct checking would cause enormous lines and delays for passengers at a time when we're trying to get people back on airplanes and get the airlines healthy again," Stempler said. "Long lines at airports would have turned a lot more people away from air travel."

The baggage searches could also contribute to delays. For example, passengers will have to be present during hand searches of their luggage, Denver airport spokesman Chuck Cannon said. Passengers will be taken to private rooms or screened-off areas for such searches.

"If the result is a slight increase in security and a huge increase in passenger processing times, it's going to be detrimental," said aviation consultant Nick Lacey, a former director of flight standards for the Federal Aviation Administration.

The stepped-up security after the Sept. 11 terrorist attacks resulted, at least initially, in long lines and hours-long delays for travelers.

Under the new law signed by President Bush in November, airlines will be responsible for security until Feb. 17, when the burden shifts to the federal government.

Airlines, airports and the government also are determining how best to meet a year-end deadline in the law to screen all baggage for explosives with $1 million, van-size explosives-detection machines. About 160 are in use, and at least 2,000 more will have to be bought by the government, FAA officials have said.

San Francisco airport spokesman Ron Wilson said all airlines at his airport have indicated they plan to use bag matching to meet Friday's deadline. "It's either do that or don't fly," he said.

Kevin Dillon, director of New Hampshire's Manchester Airport, warned of the danger of relying too heavily on baggage screening.

"We should also be focusing on other things: immigration laws, passenger profiles, interrogation techniques," he said. "There are so many things this country needs to be looking at in terms of aviation security."