On August 9, the U.S. government proposed a scaled back plan to upgrade computerized airline passenger screening using personal information, a long delayed centerpiece of aviation security.

Homeland Security Secretary Michael Chertoff said officials believe they have resolved congressional and other concerns about personal privacy and were ready to begin testing with airlines this fall.

"This information will better identify individuals who may pose a known or suspected threat to aviation or national security," Chertoff said at a news conference at Washington Reagan National Airport.

More than 600 million passengers fly on domestic airlines annually.

One year after authorities said they had disrupted a plot to blow up transatlantic flights using liquid explosives, Chertoff said threats to the United States and to air travel remain. Chertoff said there is no information pointing to specific plans for an attack but said vigilance was crucial.

"This is a very live issue," Chertoff said.

Airlines remain at the nation's second-highest color-coded threat level -- orange.

Under the revised program for domestic travel, dubbed Secure Flight, passenger names would be run through U.S. security watch lists. Passengers could be prevented from boarding while others that raise suspicion could be singled out for extra security.

Airlines would submit information electronically to the Transportation Security Administration 72 hours before takeoff, but names could be screened up until takeoff, security officials said.

Passenger names and flight itinerary would be required. Airlines would have to ask for address, gender and birth date information, but passenger compliance for that data would be optional.

Authorities said, however, that providing the voluntary information would be to passengers' advantage because it would accelerate screening, enhance security and reduce chances that travelers would be singled out for extra checks.

In another step, the U.S. government plans to require overseas airlines transmit passenger data to U.S. Customs officials 30 minutes before departure, rather than the current requirement allowing checks until the plane leaves the gate.

Authorities say earlier vetting will eliminate the need to divert or turnaround U.S.-bound airliners because of suspicious passengers, which occurs occasionally.

Major U.S. airlines reacted cautiously, saying through their trade group that the program must be "carefully crafted" to ensure privacy and minimize any impact on operations. The Air Transport Association said it was essential government weigh carrier feedback during the public comment period.

TSA hopes to have the program in place by 2008.

Attempts since 2003 to develop a new pre-screening system ran into trouble and were scrapped. Government and other watchdogs said the efforts were incomplete and TSA was criticized for overreaching with plans to use commercial and more detailed personal information. Those proposals alarmed privacy advocates and angered Congress, which ordered changes.

TSA will test the program with data from airlines that volunteer to participate. Currently, airlines are responsible for checking passenger names against government security lists before takeoff.

Homeland security officials have halved the size of various watch lists to simplify vetting and reduce chances that travelers will be mistakenly identified as potential threats -- an embarrassing problem for TSA and airlines in the past.