The first test in almost two years of the planned multi-billion dollar US anti-missile shield has failed.
The Pentagon blamed the test failure on an "unknown anomaly"
The Pentagon said an interceptor missile did not take off and was automatically shut down on its launch pad in the central Pacific.
A target missile carrying a mock warhead had been fired 16 minutes earlier from Kodiak Island in Alaska.
The Pentagon is spending $10bn a year on the missile system, which was meant to be in operation by the end of 2004.
The Missile Defence Agency said an "unknown anomaly" was to blame for the system shutting down.
A spokesman said officials would now study data from the launch site at Kwajalein Atoll, in the Marshall Islands, to establish what went wrong.
In earlier tests, target missiles have been successfully intercepted in five out of eight attempts.
Wednesday's trial had been put off four times because of bad weather at launch sites and, on Sunday, because a radio transmitter failed.
A Pentagon spokesman told Reuters news agency the test had not been tied to the question of when the national missile defence system would be declared operational.
Philip Coyle, chief weapons tester under former US President Ronald Reagan, told Reuters: "This is a serious setback for a programme that had not attempted a flight intercept test for two years."
The goal, announced by US President George W Bush in 2002, was to have a basic ground-based shield in place by the end of this year.
The last test, in December 2002, failed when the interceptor missile did not separate from its booster rocket.
The programme has been nicknamed "son of Star Wars" after the original Strategic Defence Initiative - or "Star Wars" - outlined by President Reagan in the 1980s.