HONOLULU -- Crews searched in rough seas Monday for the pilot of a U.S. mail carrying twin-engine plane that crashed into the ocean.

Debris from the Beechcraft 1900, with only the pilot aboard, was spotted in the water by the crew of a boat from Coast Guard Station Kauai nearly four hours after the Beechcraft 1900 was last seen on radar, the Coast Guard says.

"We have narrowed down the search area to a compact debris field and will work as quickly as possible," says Kevin Conroy, a search and rescue controller with the Coast Guard's Joint Rescue Coordination Center.

Coast Guard crews reported winds of nearly 30 mph and 15-foot seas.

The cause of the crash was being investigated. Federal Aviation Administration spokesman Ian Gregor says no distress call was received before the crash.

The plane, carrying about 4,200 pounds of mail, was owned by Alpine Air, which released no information about the pilot. Alpine Air is subcontracted by Postal Service contractor Corporate Air to fly mail to several destinations within the state.

Some of the mail - likely a mix from Hawaii, the mainland and international points - was recovered and will be delivered, Postal Service spokesman Duke Gonzales said.