Washington (CNN) -- Pilot Peter Hertzak, a cosmetic surgeon, took off on a solo flight from Slidell Municipal Airport in Slidell, Louisiana, last Friday in his twin-engine Cessna 421C en route to Sarasota, Florida. Some three hours later, his plane descended in a fatal death spiral into the Gulf of Mexico. What happened in between still has federal investigators looking for clues.
The National Transportation Safety Board released a preliminary report on Friday that provided little in the way of explanation as the investigation continues.
Hertzak filed instrument flight rules and departed around 6:45 a.m. local time, the NTSB said.
Investigators say the pilot contacted the Jacksonville Air Traffic Control Center at approximately 7:15 a.m. to report on his flight altitude.
About 20 minutes later, the airplane began to "deviate from the air traffic control assigned altitude and route."
The NTSB said the air traffic control center tried unsuccessfully to contact Hertzak.
The North American Aerospace Defense Command launched military fighter aircraft to intercept the plane. According to the NTSB, the military pilots reported the airplane was "circling in a left hand turn at a high altitude, low airspeed and the windows were partially frosted over."
They reported Hertzak was "slumped over in the cockpit and not moving." The NORAD planes fired flares, but the pilot remained unresponsive.
The Cessna circled for about three hours at about 28,000 feet before it made what appeared to be a soft landing. A Coast Guard spokeswoman said the plane was afloat with its tail sticking out of the water before it soon sank, about 190 miles southeast of Mobile, Alabama.
Three days prior to the incident, Hertzak had been placed on probation and permanently banned from performing plastic surgery, according to Rita Arceneaux, spokeswoman for the Louisiana Board of Medical Examiners.
When asked if the NTSB had any reason to believe the incident could have been a suicide, NTSB spokesman Eric Alleyne told CNN that was "beyond the scope of my investigation."