WASHINGTON – An experimental interceptor missile failed to get off the ground in a test of America’s national missile defense system early yesterday, raising new doubts about prospects for the imminent activation of the system.

In the test, a target missile, a simulated ICBM with a mock warhead, was launched without problem from Kodiak, Alaska, at 12:45 a.m. EST, a statement from the Defense Department’s Missile Defense Agency said.

However, 16 minutes later, an “unknown anomaly” led to an automatic shutdown of the interceptor missile shortly before it was to launch from the Ronald Reagan Test Site at Kwajalein Atoll in the central Pacific Ocean. The target missile crashed into the ocean.

The agency gave no other details and said program officials will review prelaunch data to determine the cause for the shutdown. Most missile launching systems are designed to shut down automatically as a safety feature, but it was unclear what tripped the system, officials said.

The test was the first full test of the system in nearly two years.The Missile Defense Agency has attempted to conduct the test several times this month, but scrubbed each one for a variety of reasons, including weather problems and a malfunction on a recovery vessel not directly related to the equipment being tested.

Philip Coyle, the Pentagon’s former chief of testing who has become a critic of the Bush administration’s missile defense plans,said the cause of the failure could have been anything from a software glitch to a major hardware malfunction. He called the failure a “serious setback.”

During the test, the interceptor was not necessarily supposed to strike the target missile, officials said, but it was possible. Instead, the primary goal was to collect data on the interceptor’s performance. Yesterday’s test was also to have been the first in which the interceptor used the same booster rocket that the operational system would use.

Because it didn’t fire, it is possible the interceptor could be used again in a future test. The $85 million test was not immediately rescheduled, nor was it clear whether it would affect the follow-on test, scheduled for the spring.

Two previous tests scheduled for this year were delayed due to technical problems. In earlier testing, which critics derided as highly scripted, the interceptors went five-for-eight when launched with the goal of hitting target missiles.

It was unclear how the failure would affect plans to put the missile defense system on alert sometime in the next two weeks. The Bush administration had made it a goal to activate the system by the end of 2004.