VANDENBERG AIR FORCE BASE, Calif. >> The U.S. missile defense system can’t be relied on to hit its targets even in scripted tests a decade after it was declared operational and despite $40 billion spent, according to a newspaper’s investigation.

The Los Angeles Times found that officials overstated the reliability of the Ground-based Midcourse Defense system, or GMD.

The missile shield system was supposed to protect Americans from attacks by “rogue states” like North Korea and Iran, but it has failed in half of the tests conducted to determine its ability to intercept a mock enemy warhead. In fact, despite years of vows to fix technical shortcomings, the system’s performance has grown worse since testing began in 1999.

Dean A. Wilkening, a physicist at Lawrence Livermore National Laboratory in Livermore, has called the system’s test record abysmal.

He served on a National Academy of Sciences panel that issued a 2011 report on missile defense and said at a May 28 policy conference in Washington, D.C., that GMD was a “prototype system” that “performed less well than people had hoped” and “one shouldn’t be too surprised that it does tend to fail more than you’d like.”

Philip E. Coyle III, who was the Pentagon’s director of operational testing and evaluation from 1994 to 2001 and oversaw several early test flights, said that even successful tests should be viewed skeptically because conditions are staged.

“The tests are scripted for success,” Coyle said. “What’s amazing to me is that they still fail.”

Another test is scheduled at California’s Vandenberg Air Force Base later this month. The system has 30 interceptors including four at Vandenberg and 26 at Fort Greely in Alaska.

Instead of rigorously testing the system, it was rushed into the field in 2002 under President George W. Bush. The Obama administration now supports expanding it by deploying 14 new interceptors at Fort Greely by late 2017.

A Missile Defense Agency spokesman said in a statement the agency was working to improve its reliability. The agency’s director Vice Adm. James D. Syring told a Senate subcommittee last week that the underlying problems in two recent flight-test failures had been identified and already fixed or would be fixed by year’s end.

Boeing spokesman Dexter Q. Henson said the company “remains confident in the system’s ability to defeat potential adversaries.”