The head of a key Senate subcommittee is questioning the reliability of part of the U.S. missile defense system after a failed test earlier this month.

The Defense Department maintains rocket-like interceptors in silos at Fort Greely, Alaska, and Vandenberg Air Force Base, Calif., as part of the Ground-based Midcourse Defense System, designed to shoot down incoming threats such as nuclear missiles.

An interceptor launched from Vandenberg during a July 5 test missed its target over the Pacific Ocean. Now, some lawmakers are criticizing the Pentagon’s plans to spend more than $1 billion in the fiscal year beginning Oct. 1 in part to expand the fleet of interceptors to 44 from 30.

“It’s our responsibility to ensure that those capabilities perform as advertised,” Sen. Dick Durbin, D-Ill., chairman of the Senate Appropriations Defense Subcommittee, said during a July 17 hearing. “We know the threat is real. The question is whether our defense is real.”

The panel met to hear testimony from Navy Vice Adm. James Syring, director of the Missile Defense Agency, on the agency’s fiscal 2014 budget request.

Syring said a preliminary review of the test traced the problem to something called the exoatmospheric kill vehicle — the portion of the missile that separates from the main body to “intercept” or knock an incoming projectile out of the sky.

“The kill vehicle did not separate from the third-stage booster,” he said. “It wasn’t the booster, sir. It wasn’t the guidance system. The EKV did not separate.”

Syring said he remains confident in the Boeing Co.-made system — even though it’s never actually taken out an intercontinental ballistic missile — and pledged to conduct a full evaluation of the program.

“What’s important is continued testing,” he said.

Durbin cited among his concerns the system’s mixed record of hitting targets in only 8 of 15 attempts; the high cost of testing, which runs about $215 million per exercise; and the fact that many of the interceptors aren’t operational.

Syring acknowledged that unlike the Aegis Ballistic Missile Defense System — the sea-based program that equips Navy cruisers and destroyers with the Aegis radar system and Standard Missile-3, or SM-3, interceptors — the ground-based system faced a more demanding development schedule that resulted in interceptors being deployed before testing was complete.

Durbin said the problem is a recurring one that affects other weapons systems such as the F-35 fighter jet, the Pentagon’s most expensive acquisition program.

“We’ve run into this repeatedly, whether we’re talking about the F-35 or others — keep producing even while you’re testing,” Durbin said. “We’ve reached a point now where we’re making some critical budget decisions and may not be able to afford that luxury. What troubles me is this is a system that still hasn’t been proven to be able to protect America.”

Syring said the agency is working on “flying before we buy any more” interceptors. It’s also exploring the possibility of developing a universal kill vehicle that could be used with both ground-based interceptors and sea-based SM-3s.