Airport security screeners will begin more thorough pat-downs of some passengers later this month in an effort to prevent terrorists from sneaking aboard commercial aircraft with explosives under their clothing.

The Transportation Security Administration announced yesterday that checkpoint screeners will begin training next week for the stepped-up searches that will take effect at all U.S. airports by Sept. 20. Screeners will be permitted to pat down passengers who appear suspicious because of bulky clothing -- not just those who set off the metal detector.

As part of the new procedures, screeners will also be allowed to use the inside of their hands to conduct the searches. Current policy allows screeners to touch passengers only with the back of the hand. Both the new and old policy call for female screeners to conduct searches of female passengers.

The changes are necessary "to respond to the existing threat, based on what we have seen in terms of the Russian planes and our intelligence streams," Transportation Security Administration spokesman Mark Hatfield said. "This is a prudent move to take appropriate adjustments to our procedures."

Investigators in Russia have not determined exactly how terrorists brought down two domestic commercial planes last month, killing 90 people on board. The high explosive hexogen, also known as RDX, was found at both crash sites and investigators focused on a Chechen woman on each plane. It is still unclear whether the women hid explosives in their clothing or placed bombs in suitcases or somewhere else.

Homeland Security officials last week called for more thorough passenger and luggage searches on Delta Air Lines and Aeroflot Russian Airlines' U.S.-bound flights from Moscow.

The new U.S. security procedures are the latest in a series of steps taken by the TSA to strengthen the nation's defense against the possible smuggling of explosives aboard airliners. The TSA has deployed walk-through portals that can detect explosive residue on passengers' bodies at five airports and this week began testing document scanners at Reagan National Airport that examine boarding passes for explosives.

Screening passengers and carry-on luggage for explosives was one of the 9/11 Commission's top air security recommendations. But consumer groups said more intensive personal searches might prompt angry responses from travelers, especially from female passengers who may not appreciate a closer touch from security screeners. The new procedures brought back memories of some inappropriate searches that female passengers and flight attendants complained about after the terrorist attacks in 2001, before the TSA took over screening.

"There have been concerns, mostly from young women but even older women and men, who complained that this discretion isn't the best," said James Plummer, policy analyst at Consumer Alert, a nonprofit consumer advocacy group. "Open-handed searches in close quarters and more discretion on the part of screeners really does open up the door for more complaints of that type."

The TSA said that the new procedures would be respectful. "I think there is a really broad understanding among the flying public of the reason and necessity for all our screening procedures," Hatfield said.

Frequent flier and Arlington resident Soo Cho said the extra searches are now a part of flying. "Some people say it's so invasive, but we don't want what happened [on Sept. 11, 2001] to happen again," Cho said.