An F-15C pilot was incapacitated during a rapid descent, causing the fatal Aug. 27 crash near Deerfield, Virginia, the Air Force announced Monday.

Lt. Col. Morris Fontenot, a pilot with the 104th Fighter Wing of the Massachusetts Air National Guard, was returning to his home base of Barnes Air National Guard Base, Massachusetts, from Naval Air Station Joint Reserve Base New Orleans, where his jet was receiving a radar upgrade.

An Air Force Accident Investigation Board report, released Monday, states that the Washington Air Route Traffic Control Center requested that Fontenot make a routine radio frequency change at about 8:56 a.m. He responded, "Hawk 11, declaring emergency," using his radio call sign. The air traffic controller acknowledged the statement, and asked for the pilot's status. Seconds later, the pilot responded "Affirm. Standby."

That was the last radio call.

The pilot continued a rapid descent, reaching supersonic speed, before crashing into a heavily wooded area at 8:58 a.m. At impact, he was inverted between 60 to 70 degrees nose low and at an airspeed of .83 Mach, or more than 631 miles per hour.

Investigators were unable to find a reason Fontenot was incapacitated.

"With no eyewitness accounts, surviving aircrew members, detailed emergency calls, or flight data recordings, and with minimal information from analysis of recovered (aircraft components), the specific reason (Fontenot) became incapacitated could not be determined," the report states.

The F-15C was destroyed in the crash at a loss of $45.2 million. There were no other injuries.

Fontenot was a 1996 graduate of the Air Force Academy, and an F-15 instructor pilot with more than 2,300 flight hours. He was a graduate of the U.S. Air Force Weapons School and had flown Eagles for more than 17 years.

After the crash, rescuers — including local officials on horseback, several helicopters and an HC-130 from Moody Air Force Base, Georgia — searched for two days to find the wreckage.