US Coast Guard crews have not seen any signs that the pilot of a small plane survived when his Cessna went down in the Gulf of Mexico about three hours after two F-15 fighter jets tried to make contact with him.
Coast Guard Chief John Edwards said the plane landed right-side up on the ocean surface and floating, but monitoring planes did not see a life raft deploy or anything to indicate the pilot - the lone person aboard - was alive.
The Cessna 421C later started sinking into the Gulf about 200 kilometres west of Tampa, Florida, in about 460 metres of water.
A Coast Guard cutter and rescue helicopter were en route to the crash site, but not expected to arrive before the plane was well submerged.
Federal Aviation Administration spokeswoman Kathleen Bergen said the plane was flying from Slidell, Louisiana, to Sarasota, Florida. She said controllers lost contact with the pilot at 9am (0000 AEST). The Coast Guard said it went down about 12.10pm (0330 AEST).
Authorities have not identified the pilot. But Bill Huete, a mechanic who has worked in the past on the downed aircraft at Slidell Airport, identified him as Dr Peter Hertzak, an OB-GYN who worked in the Slidell community just northeast of New Orleans.
"I met him years ago when he was looking to start flying again and bought this plane," said Huete.
Huete described Hertzak as an excellent pilot. "He flew by the book and he didn't scrimp on maintenance," Huete said.
Huete said he went to the airport after hearing reports that identified the plane and was upset by the news.
Two F-15 fighter jets tried to make contact with the plane and were flying with it and monitoring it, but weren't able to hail the pilot, said North American Aerospace Defense Command spokesman John Cornelio.
The jets' pilots reported that the Cessna's windshield was iced over and that the plane was fluctuating between 25,000 and 35,000 feet (7600 and 10,700 metres).
A computer-generated image provided by FlightAware, a private aviation flight path service, shows the plane travelling in several circles over the Gulf for hours before going down.