On January 15, 2001, a Cessna 172N, N6452D, was reported as missing. The personal flight originated from Concord, California, about 1420 hours Pacific standard time on a personal flight to an unknown destination. The pilot/owner was operating the airplane under the provisions of 14 CFR Part 91. The private pilot, believed to be the sole occupant, is missing. Visual meteorological conditions prevailed. No flight plan had been filed. As part of the search efforts for the missing airplane, a Federal Aviation Administration (FAA) specialist reviewed recorded radar data from the FAA Air Route Traffic Control Center in Oakland, California. The specialist related the recorded radar data to a record of radio transmissions between the pilot and Concord Air Traffic Control Tower. A secondary 1200 (VFR) beacon code was noted on the departure path from Concord at the time the airplane took off. The tracking started about 1420 at a mode C reported altitude of 600 feet msl (mean sea level). The target maintained a steady climb as the track generally headed in a southwesterly direction. It reached a mode C reported altitude of 10,500 feet about 22 minutes later. For the next 1 hour 20 minutes, the mode C reported altitude varied between 8,500 feet and 11, 800 feet. About 1550, the mode C reported altitude began to gradually decrease at the rate of 600 to 900 feet per minute. The last recorded target was at a mode C reported altitude of 1,000 feet at 1602:48. The specialist determined this was 36 degrees 12 minutes 37.229 seconds north latitude and 122 degrees 40 minutes 20.596 seconds west longitude. A Safety Board computer program determined this position was over the Pacific Ocean 46 miles from Monterey, California, on a magnetic bearing of 224 degrees.