Investigators are looking into the crash of a U.S. Navy destroyer and cargo ship off the coast of Japan that killed seven sailors. The USS Fitzgerald and a Philippine-registered container ship crashed early Saturday south of Tokyo Bay. The United States and Japan have since launched investigations into what might have caused the crash. U.S. Navy experts say collisions involving such ships are not common. Vice Admiral Joseph Aucoin is commander of the U.S. Seventh Fleet, based at Yokosuka naval base in Japan. Speaking to reporters Sunday, he said the crash happened at around 2:20 a.m., as many crew members slept. He added that it is Navy policy to have some crew members keeping watch throughout the night. No bad weather was reported. Aucoin said he did not know if crew members heard a warning before the crash. He also said he was not sure what conditions the seven sailors who died faced.