WASHINGTON -- Government investigators smuggled liquid explosives and detonators past airport security, exposing a dangerous hole in the nation's ability to keep these forbidden items off of airplanes, according to a report made public Wednesday. The investigators learned about the components to make an improvised explosive device and an improvised incendiary device on the Internet and purchased the parts at local stores, said the report by the Government Accountability Office. Investigators were able to purchase the components for the two devices for under $150, and they studied the published guidelines for screening to determine how to conceal the prohibited items as they went through checkpoint security. At the end of the testing, investigators concluded that terrorists could use publicly available information and a few cheaply available supplies to damage an airplane and threaten passenger safety. "It is possible to bring the components for several IEDs and one IID through TSA checkpoints and onto airline flights without being challenged by transportation security officers," said the GAO, Congress' investigative arm. The covert tests were conducted at Transportation Security Administration checkpoints at 19 airports in March, May and June of this year. The GAO did not identify the airports. In August 2006, the TSA changed its screening policies after officials foiled a plot to use liquid explosives to blow up commercial airlines headed toward the U.S. TSA spokeswoman Ellen Howe said the exercise tested only one of the agency's 19 layers of security and therefore doesn't mean the overall system is unsafe. "While people think about us in terms of the checkpoints and they see us as the checkpoints, there's a lot more layers of security," she said. In addition to the checkpoints, the TSA uses different technologies and has officials who check the validity of documents and observe people's behaviors throughout the airport. "Just because somebody gets through one layer doesn't mean they're going to get through all of the layers." The report released Wednesday is a version of a classified report with sensitive information left out. The report notes that the covert operations were intended to test only security at checkpoints and not all of the TSA's security layers. The report listed several instances in which investigators were able to make it through security checkpoints while carrying prohibited items: On March 23, a TSA screener would not let one investigator through a checkpoint with a small, unlabeled bottle of shampoo, even though it was a legitimate carry-on item. But the same investigator was able to bring through a liquid component of bomb that would start a fire. On May 8, an investigator placed coins in his pockets to ensure he would receive a secondary screening at the checkpoint. But after doing a pat-down and using an electronic hand wand, the screener was not able to catch the prohibited items the investigator brought through the checkpoint. The TSA agreed with the investigators' recommendation to introduce "more aggressive, visible and unpredictable security measures," as well as the recommendation to deploy new detection technologies.