WASHINGTON (AP) -- The Bush administration has yet to follow through on a two-year-old plan to find and plug holes in air cargo security and doesn't even have a schedule for completing it, according to congressional investigators.A report by the Government Accountability Office being released Wednesday says the Transportation Security Administration won't be able to protect cargo-carrying planes from terrorists unless it understands where they're vulnerable."According to officials, limited resources and competing priorities have delayed agency efforts to conduct such an assessment," said the GAO report, which was obtained Tuesday by The Associated Press. The GAO is Congress' investigative and auditing arm.Critics say it makes no sense to screen people and luggage carefully but not the cargo on passenger planes. Last year, about 6 billion pounds (2.7 billion kilograms) of cargo -- a quarter of the cargo shipped by air in the United States -- was flown aboard passenger airplanes.Critics also say cargo planes need to be protected because terrorists could use them as weapons. Late in 2003, Homeland Security officials said intelligence indicated al-Qaeda might hijack cargo planes and attack nuclear plants, bridges or dams.Since the September 11 attacks, several stowaways have been discovered in the holds of cargo aircraft.Air cargo loaded onto passenger aircraft must be shipped by a company that has registered with the TSA. Cargo airlines have security plans and some cargo is randomly inspected.But the report noted that the TSA collects information on less than one-third of the registered companies that ship goods on passenger planes, and that information may not be reliable.The TSA has also exempted certain kinds of cargo from inspections because it doesn't view it as a risk, the GAO said. The report noted that visits to four airports showed that "a considerable amount of cargo" being loaded and unloaded on passenger airplanes was exempt from inspection.TSA spokeswoman Yolanda Clark said the agency required airlines to triple random inspections of cargo, hired 100 cargo inspectors and is testing new security technology."TSA has established a strong layered system of security in the air cargo arena and recognizes the need to do more," Clark said.Congressman Ed Markey, a Democrat who requested the report, said it confirms the concerns he has raised about loopholes in cargo security."GAO's report blows away the Bush administration's smoke screen that paperwork checks, random inspections and other half measures keep Americans safe," Markey said in a statement.Congressman Chris Shays, a Republican who also asked the GAO for the report, said it's time to implement tougher inspection regulations."Uninspected cargo is a risk to air passengers," Shays said in a statement.The TSA, which is part of Homeland Security, promised regulations to plug holes in air cargo security by the end of 2003. Congress gave the agency an August deadline to come up with the rules. The TSA has yet to do so.Copyright 2005 The Associated Press. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.