WASHINGTON (CNN) -- Investigators with bomb-making components in their luggage and on their person were able to pass through security checkpoints at 19 U.S. airports without detection, according to the Government Accountability Office.



Passengers pass through security at O'Hare International Airport in Chicago, Illinois.



GAO officials are expected to testify about the investigation Thursday before the House Committee on Oversight and Government Reform.



The investigators reported that most of the time security officers followed Transportation Security Administration policies and procedures, but investigators were able to take advantage of "weaknesses in TSA procedures and other vulnerabilities."



"These weaknesses were identified based on a review of public information," the planned GAO testimony says.



Investigators concluded that if they had attempted the same test at other airports, they would have evaded detection.



But the GAO did not detail the weaknesses because they "are sensitive security information."



The investigators obtained the bomb-making components at local stores and over the Internet for less than $150, according to testimony.  Watch a tester point out a TSA mistake »



The GAO said its investigators also tested the devices that could be built with the components they smuggled and discovered that "a terrorist using these devices could cause severe damage to an airplane and threaten the safety of passengers."



The GAO investigators devised two types of devices: an "improvised explosive device" made of a liquid explosive and a low-yield detonator, and an "improvised incendiary device" that could be created by combining commonly available products prohibited in carry-on luggage.



The GAO said it found the instructions for creating the devices "using publicly available information," including Internet searches.



According to the testimony, a transportation security officer barred one of the investigators from bringing an unlabeled bottle of medicated shampoo through the checkpoint. But the security officer allowed a liquid component of the improvised explosive device to pass through undetected, although that item is prohibited by the TSA.



In another test, the investigator put coins in his pockets to assure he would get a secondary inspection. But the officer, using a hand-wand and a pat-down, failed to detect any of the prohibited items the investigator was carrying.



The GAO said it had briefed the TSA on its findings "to help them take corrective action."



In testimony to be provided to the same congressional committee, TSA chief Kip Hawley defends the administration's policies and procedures, saying that the screening checkpoints are but one of a "multilayered approach to security."



"We recognize that, despite our efforts to make each layer as strong as possible, a concerted effort may target any one layer," according to the testimony. "Our ongoing success is a result of the tremendous power in the reinforced, multiple layers. Truly, the whole is greater than the sum of the parts -- and together, they are formidable."



Hawley sketches for members of Congress 19 security steps the TSA employs before, during and after checkpoint screening. "Each and every one of these 19 security layers is important and strong in its own right," he says. "Linked together, they are effective and daunting."



Although it would not discuss the specific nature of its recommendations, the GAO said it recommended establishing special screening lines based on risk and passengers with special needs. The TSA should introduce more "aggressive, visible and unpredictable" measures to detect concealed items and develop new technology for screening at checkpoints.



Hawley's said the TSA concurs with the GAO's recommendations and specifically discussed several "new technology" items that he said were "greatly improving our effectiveness in detecting prohibited items."



Among the new technology, he said, were whole body imagers, bottled liquids scanners, hand-held explosives scanners and advanced technology X-rays. And, he added, "our pursuit of new technology is not limited to what I described today."



He also said the TSA is constantly conducting covert tests of the screening process, including detection of prohibited liquids and IEDs. "The nation's aviation system remains secure," he said, "but requires
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question: what did terrorists use?, answer: "improvised explosive device" | question: who took advantage of weaknesses?, answer: investigators | question: what did GAO say about terrorists?, answer: "a | question: Were the weaknesses detailed?, answer: the GAO did not detail the | question: What does GAO stand for?, answer: Government Accountability Office. | question: What did the investigators take advantage of?, answer: "weaknesses in TSA procedures and other vulnerabilities." | question: what does TSA stand for?, answer: Transportation Security Administration | question: What wasn't detailed because of security reasons?, answer: the weaknesses