An interceptor missile fired from a silo at Vandenberg Air Force Base in California in a test of the backbone of U.S. ballistic missile defense failed to hit a target missile over the Pacific on Friday, the Pentagon said.

The miss is likely to raise additional questions about the reliability of a U.S. ballistic missile defense system that the U.S. Government Accountability Office said has received $90 billion in funding since 2002.

The Missile Defense Agency, U.S. Air Force 30th Space Wing, Joint Functional Component Command, Integrated Missile Defense and U.S. Northern Command conducted the flight test of what’s known as the ground-based midcourse defense element of the nation’s ballistic missile defense system.

Mobile and ship-based missile defense systems also are part of the mix.

"Although a primary objective was the intercept of a long-range ballistic missile target launched from the U.S. Army’s Reagan Test Site on Kwajalein Atoll, Republic of the Marshall Islands, an intercept was not achieved," the Pentagon said in a release.

"Program officials will conduct an extensive review to determine the cause or causes of any anomalies which may have prevented a successful intercept," the Pentagon said.

The $2 billion Sea-Based X-Band Radar, which is normally berthed at Pearl Harbor’s Ford Island but left port weeks ago, was to provide tracking for the test, officials said.

Ground-based interceptors have hit their targets in just eight of 16 tests, with the most recent successful test in 2008.

"This is a pretty important test. If you look at the stats, it’s been quite a while since we’ve had a successful intercept test," Riki Ellison, chairman of the Missile Defense Advocacy Alliance, said last month.

The group advocates for a strong national ballistic missile defense program.

The interceptor missile carried what’s known as the CE-1 "exo-atmospheric kill vehicle," which is designed to slam into and destroy an enemy ballistic missile.

George Lewis, a missile defense expert, said on his website, mostlymissiledefense.com, that the CE-1 began deployment in 2004 and that the Missile Defense Agency began development of a new interceptor, the CE-2, in 2005.

The CE-1 kill vehicles successfully hit targets in flight tests in 2006, 2007 and 2008, Lewis said, while the CE-2 version failed in two intercept tests in 2010.

Controversy surrounds ground-based missile defense not only due to its reliability in technologically complex tests likened to trying to hit a bullet with a bullet, but also because of the additional challenges that likely would exist with decoy warheads and other countermeasures in incoming missiles.

Despite those challenges, Defense Secretary Chuck Hagel announced March 15 that to "stay ahead of the challenges posed by Iran and North Korea’s development of longer-range ballistic missile capabilities," the United States was adding 14 more ground-based interceptors in Alaska.

Twenty-six interceptors are in silos at Fort Greely, Alaska, and four others had been at Vandenberg.

Adm. James Winnefeld, vice chairman of the Joint Chiefs of Staff, said at the same time that "we have confidence" in the CE-1 missile and that fixes to the CE-2 variant — still to be tested in intercepts — will prove it to be successful, as well.

"We retain our confidence in the CE-1 missile, which is in silos up in Alaska right now," Winnefeld said. "So the American people should have faith in that missile and that we can defend ourselves against a potential North Korean threat as it exists today."

Bruce MacDonald, a former assistant director for national security at the White House who was part of a ballistic missile defense panel discussion last month, said in his presentation slides that the United States can’t count on its defensive systems working even reasonably well.

Even so, they are a deterrent for foreign powers, better than no defense at all and shouldn’t be ignored, MacDonald said.