KUWAIT CITY, Kuwait (CNN) -- A U.S. Marine was killed and another was injured in a shootout Tuesday on Failaka Island, an official from the U.S. 5th Fleet said.

Two civilians were also killed, according to U.S. military officials.

The shootout began when two people in a civilian vehicle fired on U.S. troops, Kuwait Defense Ministry officials said. There was an exchange of gunfire.

The Marines' training areas on Failaka island are normally sealed off from civilian access to provide security for the U.S. troops.

It is not clear how a civilian vehicle got into the area.