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WASHINGTON—A top secret U.S. Navy acoustic detection system designed to spot enemy submarines first heard the Titan sub implosion hours after the submersible began its mission, officials involved in the search said. The Navy began listening for the Titan almost as soon as the sub lost communications, according to a U.S. defense official. Shortly after its disappearance, the U.S. system detected what it suspected was the sound of an implosion near the debris site discovered Thursday and reported its findings to the commander on site, U.S. defense officials said. “The U.S. Navy conducted an analysis of acoustic data and detected an anomaly consistent with an implosion or explosion in the general vicinity of where the Titan submersible was operating when communications were lost,” a senior U.S. Navy official told The Wall Street Journal in a statement. “While not definitive, this information was immediately shared with the Incident Commander to assist with the ongoing search and rescue mission.” The Navy asked that the specific system used not be named, citing national security concerns.