The purportedly successful 1997 test of the nation’s controversial missile defense system may have been a failure, the U.S. General Accounting Office (GAO) has said. The test has long been heralded as demonstrating the potential for a working missile defense shield.

Missile defense tests are carried out at Vandenberg Air Force Base in North Santa Barbara County and U.S.-owned islands in the South Pacific. During each test, a defensive missile is fired from a South Pacific island to intercept dummy warheads launched from Vandenberg.

The GAO announced that it will release a report detailing the failure of an infrared sensor onboard the defensive missile. The sensor was designed to operate at a temperature of -263 degrees centigrade, but during the test the temperature never reached this low extreme. In an attempt to correct the problem, controllers attempted to recalibrate the sensor in mid-flight, which only exacerbated the problem.

Problems with the test have been ignored in the past years because the sensor failure was never discussed in unclassified documents, GAO officials said. The GAO report has not yet been released, but some scientists claim that it gives credence to claims by MIT physicist Theodore Postol that the government intentionally concealed evidence of test failures.

Officials at the Pentagon have declined to comment, saying only that the sensor in question has been replaced with a more accurate counterpart in newer models of the missile defense system.