The Defense Inspector General's Office will look into whether the defense secretary improperly shared sensitive information on the commercial app.
The U.S. continues to strike Houthi militants, who have targeted Israel and American ships in the region.
Republican and Democratic leaders on the Senate Armed Services Committee are requesting a formal investigation into the Signal chat scandal.
Former U.S. defense officials said the unsecured messaging chain could have jeopardized the operation and put American lives at risk.
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