WASHINGTON (CNN) -- Pirates off the eastern coast of Africa fired on U.S. sailors Saturday as they tried to reach the lifeboat where an American captain is being held, a U.S. official familiar with the situation told CNN. The guided missile frigate USS Halyburton, with helicopter capabilities, is now at the scene. The gunfire forced the sailors, who did not return fire, back to the guided missile destroyer USS Bainbridge, the official said. Capt. Richard Phillips reportedly offered himself as a hostage to the pirates during an attack Wednesday on the U.S.-flagged container ship Maersk Alabama in the Indian Ocean.
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NEW: Gunfire from pirates forces sailors, who did not return fire, to turn back
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NEW: FBI launches criminal investigation into hijacking, hostage-taking
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Maersk Alabama arrives in Mombasa, Kenya under armed guard
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Capt. Richard Phillips is being held by four gunmen in covered, fiberglass lifeboat
Pirates have been searching the waters off Africa's coast for the @placeholder's lifeboat, a U.S. military official with knowledge of the situation said Friday.