WASHINGTON - By midnight tonight, all bags checked onto passenger planes will be screened for explosives, most by electronic machines, with trained dogs and hand searches used temporarily at a few airports, the head of the Transportation Security Administration said yesterday.

With that, said Adm. James M. Loy, who as undersecretary of transportation for security is in charge of the fledgling agency, the last major deadline imposed by Congress to improve airport security after the Sept. 11 attacks will be met.

"This is a tremendous success, a credit to our screeners, to the airports and the airlines, our contract partners Boeing-Siemens, and the suppliers who provided the electronic equipment that we're installing around the country," he said. "They and literally thousands of others have worked incredibly hard to reach a milestone that makes every airport in our nation, and everyone who flies, safer than they have ever been."

Still, the goal of full screening for checked bags, which was the last of 36 mandates set by Congress when it established the TSA last year, was met with certain asterisks, with some airports still scrambling to install electronic scanners and integrate them into their baggage handling systems.

In the airports that cannot provide full coverage with electronic scanners, transportation security officials will rely on explosives-sniffing dogs, manual searches and last-minute matching of bags with passengers to meet the requirement of 100 percent screening, Loy said.

He said that more than 90 percent of bags at the nation's 429 commercial airports could now be screened electronically, up from 5 percent 15 months ago.

In some cases, the screening machines, which are the size of a minivan, sit in temporary locations, as workers prepare permanent sites with adequate power and access to conveyors. The ad hoc arrangements have strained already cluttered airport lobbies and required travelers to wait in as many as three lines before boarding.

Officials also acknowledge that the screening machines can generate false positive readings for explosives as much as 30 percent of the time. In such cases, security officials inspect the bags by hand.

Loy declined to say which airports had received waivers that give them more time to install the screening devices, which are similar to the equipment used for CT scans in hospitals, or to use a combination of other screening techniques. He said "a handful of airports," including five of the largest, would finish construction by March at the latest.

Whatever the shortcomings, the full baggage screening marks the culmination of a harried and ambitious year for the agency, which grew from 13 employees in January to about 64,000 full-time and part-time workers, of whom about 56,000 screen passengers and baggage.

The agency has placed thousands of federal air marshals on daily flights, stiffened rules for perimeter security at airports and fortified cockpit doors.

Last month, the agency cleared a major hurdle by announcing that it had hired, trained and deployed federal screeners at all of the nation's airports.