(CNN) -- A small plane with an unresponsive pilot sank in the Gulf of Mexico on Thursday after circling above the ocean for more than two hours, then crashing, the U.S. Coast Guard reported.
The twin-engine Cessna 421 remained afloat with its tail sticking out of the water for some time before it sank Thursday afternoon, said Petty Officer Elizabeth Bordelon, a Coast Guard spokeswoman. No remains have been found, and the Coast Guard still considered its operation an active search and rescue. There was no sign of the pilot, Bordelon said.
The plane went down about about 120 miles west of Tampa, Florida, at 12:08 p.m. after circling the eastern Gulf for more than two hours, said Chief Petty Officer John Edwards, a Coast Guard spokesman. The crew of a Coast Guard search-and-rescue plane watched as the Cessna made what appeared to be a soft landing, Edwards said.
A Coast Guard plane from the Tampa area and the cutter Coho were still taking part in the search Thursday evening, and the Coho was expected to remain at the scene overnight, Bordelon said.
The plane took off from Slidell, Louisiana, en route to Sarasota, Florida, with a single pilot on board. It had been circling at an altitude of about 28,000 feet, a Federal Aviation Administration source told CNN. The Air Force began monitoring the plane after noticing it flying erratically over the Gulf of Mexico on Thursday morning, and planes sent up to investigate it reported the Cessna's windows were either iced or fogged over, Edwards said.
Pilots at the Slidell Municipal Airport said the victim was Dr. Peter Hertzak, a cosmetic surgeon who was well known in the area.
"I've lost a dear friend, just a great guy," pilot Bill Huete told CNN affiliate WWL-TV. "Super doctor and just a regular guy that would do anything for anybody."
Hertzak was the head of the Hertzak Laser Center, which specializes in liposuction and cosmetic surgery. He had more than 30 years of experience, according to the center's website.
The Coast Guard plane and two F-15s from the North American Aerospace Defense Command were still watching the Cessna before it went down, the Air Force and Coast Guard reported.
The Coast Guard said an investigation was under way. But aviation expert Miles O'Brien told CNN the circumstances point toward a possible loss of cabin pressure in the eight-seat, propeller-driven aircraft.
"At 28,000 feet, you don't have an awful lot of useful consciousness without the support of oxygen or being in a pressurized aircraft," said O'Brien, a former CNN correspondent. If a lone pilot is incapacitated at that altitude, "there aren't a lot of options for resuscitating him and getting him back flying."
Fellow pilot Patrick Quigley told WWL that Hertzak maintained his plane meticulously and spared no expense to keep it in "tip-top shape."
Quigley said Hertzak was an accomplished pilot and that the flight should have been routine, "but when something goes wrong, obviously you have nowhere to go when you're out over the ocean."
In 1999, a private jet carrying golfer Payne Stewart and five others crashed after apparently losing cabin pressure "for undetermined reasons" after takeoff from Florida, the National Transportation Safety Board found.
Fighter pilots were sent up to intercept Stewart's plane after controllers lost contact with it, and they reported its cockpit and cabin windows were frosted over.
The plane flew more than halfway across the United States, apparently on autopilot, until it crashed in a South Dakota field.
CNN's Dave Alsup, Rick Martin, Devon Sayers, Aaron Cooper, Todd Spoerry, and Joe Sutton contributed to this report.