YOKOSUKA, JAPAN, JUNE 21 -- The U.S. Navy opened an investigation today into what caused a fire and explosions that killed two crewmen and injured 16 aboard the aircraft carrier Midway.

"The exact cause of these explosions is now under investigation," Rear Adm. Lyle Bull of the 7th Fleet told reporters on board the warship after it returned to its home base here.

The Japanese government called on U.S. military authorities in charge of the 50,000 U.S. troops based in the country to tighten safety procedures. Wary of growing anti-U.S. sentiment among residents near American bases, it requested a full and prompt report on the cause of the accident.

An initial explosion in a fourth-deck storeroom shortly after midday Wednesday killed two men and injured nine, Bull said. Seven other crewmen were injured in a second explosion 45 minutes later in the same storeroom.

The charred bodies of two sailors were discovered early today when the intense heat cooled sufficiently for a search team to enter the compartment. The bodies were taken to the Yokosuka base hospital for autopsies.

In Washington, the Defense Department identified the victims as Seaman Ulric Patrick Johnson, 20, of Martinez, Calif., and Fireman Jeffrey Allan Vierra, 20, of Nevada City, Calif.

Washington Post Staff writer Patrick E. Tyler added in Washington:

Senior officers on the Midway have relayed suspicions that the fire and explosion may have been caused by a faulty steam line that passes through the storeroom, where fuel oil and flammable liquids were stored. The steam line is part of the catapult plane launcher.

The deaths and serious injuries were thought to have resulted when fresh air rushed into the compartment and touched off a blast that was fueled by flammable vapors and the steam line, Midway officers suspect, according to Navy sources.