WASHINGTON (CNN) - The Transportation Security Administration -- famous for its role in screening all airline passengers and their luggage -- needs to develop better ways to check cargo, most of which goes uninspected into the bellies of planes, the Government Accountability Office said in a new report.

The report, released Wednesday, said the TSA has yet to assess vulnerabilities in the air cargo system.

Homeland security experts say air cargo remains one of the biggest concerns in passenger planes. Last year, an estimated 23 billion pounds of air cargo was transported within the United States, with passenger planes moving a quarter, or 6 billion pounds, of it.

But only a "very small percentage" of cargo is inspected, the GAO report said. The exact percentage is classified.

In 1988, a bomb in a suitcase in the cargo hold of Pam Am flight 103 detonated over Lockerbie, Scotland, and the resulting crash killed 270 people, including 11 on the ground.

The TSA, claiming it is not technically feasible to inspect all cargo without hurting commerce, has worked to minimize random inspections and focused instead on targeting high-risk cargo. The agency would subject cargo deemed high risk to physical inspections or nondestructive searches, such as X-rays.

The GAO has endorsed the TSA's risk-based management approach but said in the report that the agency has not yet established a method or schedule for completing assessments of air cargo vulnerabilities.

The GAO report said:

-- The TSA database of known shippers (people and businesses that routinely ship cargo) may not be reliable.

-- The TSA has exempted some cargo from inspection requirements, because of its nature and size. That could create "security weaknesses."

-- The TSA has not developed ways to measure whether shippers and airlines are complying with rules.

"Americans are really playing cargo roulette every time they get on a passenger plane," said Rep. Edward Markey, D-Mass., a frequent critic of the TSA. Cargo should go through the same scrutiny that passengers and luggage go through, he said.

"The Bush administration has given a pass to the cargo industry," Markey added.

The TSA defended its record.

"TSA has established a strong, layered system of security in the air cargo arena and recognizes the need to do more," TSA spokesperson Yolanda Clark said in a statement.

"In the last year alone, TSA has required passenger air carriers to triple random inspections of cargo, hired an additional hundred cargo inspectors to ensure compliance, launched a risk-based approach to provide additional vetting of indirect air carriers, and is testing new technologies to more effectively inspect the full spectrum of cargo."

An upcoming air cargo rule -- regulating shippers and airlines -- "will continue to raise the bar on security while balancing the needs of commerce," Clark said.

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