Transcript for Urgent search underway for 7 missing sailors

We do want to turn to the urgent search under way at this hour in what is called the worst accident in years for the U.S. Navy. The uss Fitzgerald colliding with a cargo ship off the coast of Japan. Three sailors injured and seven still missing. The damaged ship towed back to its home port and investigators piecing together what caused that crash. Matt Gutman has the latest. Reporter: Tonight us ships and aircraft scouring the busy waters off Japan for those seven missing sailors. But officials say they could also still be trapped in the wreckage on board the ship. We've still got an active search and rescue going on for our missing shipmates. Reporter: Now, a mystery emerging -- multiple maritme trackers show the acx crystal cargo ship hooking this giant u-turn Friday night minutes before nearly t-boning the uss Fitzgerald. ABC news has reached out to the owners but so far no response. Three crew members, including the ships commanding officer medivaced, others treated on board. The collision with the 30,000 ton acx Cyrstal in one of the world's busiest shipping lanes seemed to gut the ship's midsection. But the Navy said there was also damage below the water line. The damage is consequential and will probably take the Fitzgerald out of service for months if not years to come. Reporter: This evening the Fitzgerald limped back to port nudged by tugboats listing to its right side. Those hoses pumping sea water overboard. This ship, whose missile defense system has become more vital with recent north Korean provocation. This is a bad mishap for the us Navy because the Fitzgerald is one of the most sophisticated ships in the us Navy. Reporter: Probably the biggest question is how did a U.S. Navy ship with some of the most sophisticated radar equipment on the planet fail to see a nearly city-sized ship bearing down on it especially after it hooked that very conspicuous u-turn. Thank you. We want to turn to the white

This transcript has been automatically generated and may not be 100% accurate.