Washington (CNN) -- A U.S. missile defense test failed Sunday when a long-range missile missed its target because of radar problems.

The Defense Department said a target missile was launched from an Army test site at Kwajalein Atoll in the Marshall Islands at 7:40 p.m.

Six minutes later, a second missile was launched from Vandenberg Air Force Base in California, to find the first one and destroy it.

Both missiles flew successfully, but missed each other because of a problem in the sea-based X-band radar, the department's Missile Defense Agency said.

The X-band radar sits atop a modified floating oil platform and provides information about incoming missiles so military officials can launch a response.

"Both the target missile and ground-based interceptor performed nominally after launch. However, the sea-based X-band radar did not perform as expected," the agency said.

Officials intend to investigate the cause of the interception failure.