U.S. Naval Forces Central Command said in a statement that no one was hurt in the pre-dawn incident Thursday involving the nuclear-powered USS Jacksonville.
The submarine surfaced from periscope depth to see if the collision damaged the vessel, but the ship continued "on a consistent course" offering no indication of distress or acknowledgement of a collision.
According to a U.S. Navy official, the submarine was submerged when there was a "thump." When submariners tried to see what happened, they were unable to raise the periscope. When they raised the other periscope, they realized the first one had been sheared off and that a vessel - believed to be a fishing trawler - was motoring away and appeared to be operating normally, the official said.
The official, who was not authorized to discuss an ongoing investigation and spoke on condition of anonymity, said the vessel did not appear to be in any distress. The Navy tried to contact the vessel and determine what it was and what country it belonged to, but was unable to do so. The Navy also reviewed surveillance of the area later and found no vessels in distress, he official said.
The Navy said in its statement that one of the Jacksonville's two periscopes was damaged. A P-3 Orion aircraft searched the area but saw no debris in the water or vessels in distress, officials said.
The Jacksonville is currently deployed to the Middle East, conducting normal patrol operations in the gulf.
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