It was the latest chapter of an emotional debate over measures to protect commercial aircraft from terrorist attacks.

Currently, all passenger baggage is screened for explosives and a certain percentage of cargo is scanned, although the exact figure has not been revealed for security reasons.

However, Massachusetts Congressman Ed Markey and others are incredulous that passengers still fly aboard planes containing any percentage of cargo that is not checked.

"The technology already exists," he said. "The Israelis screen cargo. The United Kingdom screens cargo. The Netherlands screens cargo. Australia, Singapore, Spain, Hong Kong, Italy, they already screen the cargo which goes on to passenger planes and there are American companies lined up to do the job."

Mr. Markey and others referred frequently to Pan Am Flight 103, brought down in 1988 over Scotland by an explosives-laden package in its baggage compartment. Families of Flight 103 victims support efforts in Congress to require total mandatory cargo screening.

But lawmakers opposing the amendment maintain the technology for 100 percent cargo screening is not yet available and say implementing such measures would put airlines in danger of financial collapse.

Harold Rogers is a member of the House Appropriations Committee.

"We agree that 100 percent of all cargo on passenger planes should be screened, we are together on the goal," he said. "It's just that we don't have the capability now. We can't do it this year. It's going to take a little bit longer. The machinery doesn't exist, contrary to what Mr. Markey says."

John Mica is a Florida Republican who has been behind efforts in Congress to protect passenger aircraft from ground-to-air missiles. He argues that risks are greater of terrorists trying to get aboard planes than from explosives placed in cargo.

"We do not have the technology to address our greatest risk which is people strapping on explosives to themselves and walking through a 1950's metal detector that does not detect explosives," he added.

But Congressman Christopher Shays, a Republican from Connecticut who supports cargo screening, says Americans are under a false impression they are safe on commercial planes. "If six planes are blown out of the sky, a week from now, or two weeks from now, are all of us supposed to go back into our [congressional] district[s] and say we couldn't afford to do it?" he asked.

The Homeland Security bill contains $896 million more than requested by President Bush for the Department of Homeland Security, including money to improve screening of air passengers and cargo, including containers at land and sea border points, as well as money for research on anti-missile devices for commercial aircraft.

Debate on Homeland Security legislation came as the House moves to complete remaining spending bills, including one authorizing about $400 billion dollars in defense spending for 2005.