As soon as Navy personnel lost contact with the unmanned vehicle, a piloted aircraft was dispatched to Maryland's Eastern Shore, where it came upon the wreckage and determined that it was unlikely anyone on the ground had been hurt, Navy officials told CNN.

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Civilian personnel in New York and Washington who came in contact with the opened letters remain asymptomatic, authorities said.

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Staff Sgt LORENTO SPITERI, Armed Forces of Malta: Once we do make visual contact with the vessel we'll ID the number of personnel on board, and we'll usually immediately commence a search for about 25 to 40 square miles.

BBC: Last Updated: Wednesday, 16 January 2008, 12:19 GMT