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A small plane whose pilot either passed out or died flew in circles for several hours before running out of fuel and spiraling into the Gulf of Mexico Thursday afternoon as F-15s circled above.

The Cessna 421 is in the water about 120 miles west of Tampa, Fla., and is completely submerged, the U.S. Coast Guard said. The plane made a soft landing and was fully intact before sinking.

It is unclear how many people were on board the aircraft. Coast Guard crews saw no signs Thursday 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 had been floating right after the crash. Planes did not see a life raft deploy or anything to indicate the pilot -- the lone person aboard -- was alive, the Coast Guard said.

The Cessna 421C later sank into the Gulf about 120 miles west of Tampa, Fla., in about 1,500 feet of water. A C130 and a helicopter were looking for debris from the submerged plane, said Petty Officer Elizabeth Borderland. A Coast Guard cutter would remain in the area all night, she said.

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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 9 a.m. (1400 GMT). The Coast Guard said it went down about 12:10 p.m. (1710 GMT).

Authorities have not identified the pilot. But Bill Huete, a mechanic at the Slidell Airport, said Dr. Peter Hertzak, an OB-GYN who worked in the community just northeast of New Orleans, was the only person who piloted the plane. Huete said the doctor's wife told him her husband was flying the plane that morning.

Huete had worked on the plane for Hertzak and knew the doctor and his family for years.

"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.

No one answered the door at an address listed for Hertzak and calls to the home rang unanswered. Hertzak's office said any statement would have to come from his family and the Coast Guard said it wouldn't release the name for 24 hours according to its policy.

"This is a joint response with the Coast Guard and Air Force," said Kevin Robb, the command duty officer for the 8th District command center. "We're saturating the scene, responding with multiple air and surface assets."

Two F-15s had been scrambled after it appeared its pilot, flying about 150 miles off Florida, either passed out or died, a law enforcement official told Fox News. 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.

North American Aerospace Defense Command (NORAD) said two F-15s were scrambled to follow the Cessna 421 plane at approximately 8:45 a.m. to monitor the flight after the Federal Aviation Administration lost radio contact with the pilot.

NBC-2 reports that the plane began making a corkscrew pattern about half way through the trip.

The incident brought to mind the October 1999 death of golf superstar Payne Stewart. Months after winning the U.S. Open, Stewart, 42, was aboard a Learjet flying from Orlando to Dallas when the plane lost cabin pressure. Authorities believe everyone aboard died quickly, but the plane flew north across the U.S. for several hours before running out of fuel and crashing into a South Dakota field.

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The Associated Press contributed to this report.