(CNN) -- A small plane with an unresponsive pilot crashed in the central Gulf of Mexico on Thursday after circling above the ocean for more than two hours, but it appeared intact after hitting the water, the U.S. Coast Guard reported.
The tail of the twin-engine Cessna 421 remained sticking out of the Gulf about 120 miles west of Tampa, Florida, after it went down at 12:08 p.m. ET, 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 helicopter and the cutter Coho were expected to reach the site Thursday afternoon, he said.
The plane took off from Slidell, Louisiana, en route to Sarasota, Florida, with a single pilot on board, and 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.
Mike Maddox, a manager at the Slidell airfield where the plane took off, confirmed there was a situation with a plane and said family members had been notified, but he had no further comment.
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."
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 and