The U.S. military aborted an attempt to shoot down an incoming missile with two interceptors Wednesday after the target malfunctioned shortly after launch off Kauai.

It was the first breakdown after five successful tests of the Terminal High Altitude Area Defense system, or THAAD. Four of the successful test had been conducted with launches from the Pacific Missile Range at Barking Sands and one from White Sands, N.M.

The Missile Defense Agency says because of the failure, the two THAAD interceptor missiles that were to take part in the test were not launched. So the full test was never completed.

It was the second time a target missile had failed, resulting in a scrapping of the interceptor launch, leaving the test missiles available for future tests. The first failure was early on in the tests, which began at White Sands in 2005.

"THAAD has never had an unsuccessful attempt at an intercept," said agency spokeswoman Pamela Rogers. She said the agency will investigate the cause of the malfunction.

Rogers said the target missile was launched at 10:05 p.m. EDT from a mobile platform on the decommissioned USS Tripoli helicopter carrier.

"Because of the target malfunction, the target did not have enough momentum to reach the open ocean area previously approved for safe intercept," she said, adding that the target missile fell into the sea within the safe area.

A Navy ship equipped with Aegis ballistic missile tracking technology followed the missile. It was to relay data to the flight test detachment of the Army's 6th Air Defense Artillery Brigade on Kauai, which was to spot the target and fire the dual interceptors.

Using two interceptors increases the probability of success because the second projectile still has a chance of making contact if the first one misses.

The target was a separating one, meaning the interceptors would have had to differentiate between the target missile's warhead and booster when homing in.

The air defense artillery brigade of about a dozen soldiers flew to Hawaii from their Fort Bliss, Texas, base to carry out the test.

Like Patriot anti-missile defenses, THADD is designed to knock out ballistic missiles in their final minute of flight. Unlike the Patriot system, however, it is designed to intercept targets at higher altitudes, enabling it to defend a larger area.

Even so, it can only target short and medium-range missiles. Intercontinental ballistic missiles are out of its range.

The Army is setting up a unit, the Alpha Battery, 4th Air Defense Artillery Regiment at Fort Bliss that will operate the THAAD system. Commanders expect to be able to deploy the battery anywhere once it is completely fielded in late 2010.

THADD is one of two missile defense systems the military tests at the Pacific Missile Range Facility on Kauai. The other is the sea-based Aegis system. The Missile Defense Agency coordinates U.S. missile tests in cooperation with the Army, Navy and Air Force.