* Test was a repeat of a failed exercise in January

* No preliminary explanation of failure provided

* System is sole US defense against long-range missiles (Updates throughout, adds companies involved)

WASHINGTON, Dec 15 (Reuters) - A test of the sole U.S. defense against long-range ballistic missiles failed on Wednesday, the second failure in a row involving the system managed by Boeing Co BA.N, the Defense Department said.

“The Missile Defense Agency was unable to achieve a planned intercept of a ballistic missile target during a test over the Pacific Ocean today,” Richard Lehner, an agency spokesman, said in an e-mailed statement. No preliminary explanation of the failure was provided.

The miss brought the so-called ground-based midcourse defense’s batting record to eight intercepts out of 15 tries, as reckoned by the Missile Defense Agency.

“This is a tremendous setback for the testing of this complicated system,” Riki Ellison, head of the Missile Defense Advocacy Alliance, a booster group, said in a statement. He said it raised troubling questions about the reliability of the 30 or so interceptor missiles deployed in silos in Alaska and California.

The test was a repeat of a Jan. 31 exercise in which an advanced sea-based radar had not performed as expected.

In the test on Wednesday, an intermediate-range ballistic missile target flew successfully from a test site on Kwajalein Atoll in the Marshall Islands, as did a long-range interceptor launched from Vandenberg Air Force Base in California, the agency said.

The sea-based X-Band radar and all sensors performed as planned, and the interceptor successfully deployed a “kill vehicle” designed to collide with the target, the statement said.

It said officials will conduct an extensive investigation to pin down the cause of the failure to intercept. The next flight test will be determined after the failure’s cause is identified, it added.

A Boeing spokeswoman did not immediately respond to a reqeust for comment.

The multibillion-dollar ground-based bulwark is designed to shoot down a limited number of long-range ballistic missiles that could be tipped with chemical, biological or nuclear warheads. The system is part of a layered hedge against countries such as North Korea and Iran.

It networks systems on land, at sea and sensors in space and is meant to counter ballistic missiles of all ranges. The United States has spent more than $10 billion a year on a range of missile defense programs in recent years.

In October, a converted Boeing 747 jumbo jet equipped with a chemical laser failed to knock out a target ballistic missile over the Pacific, marking that system’s second such failed intercept test in a row. The flying laser has been scaled back to a kind of science experiment, no longer a development program aimed at eventual deployment.

Boeing's chief subcontractors on the ground-based midcourse defense include Raytheon Co RTN.N, Northrop Grumman Corp NOC.N and Orbital Sciences Corp ORB.N.

A team led by Lockheed Martin Corp LMT.N and Raytheon is competing to oust Boeing next year and take over continued development, manufacturing, test, training, operations support and sustainment of the ground-based defense. The contract is worth about $4.2 billion over seven years.