John F. Kennedy Jr. probably became disoriented during a nighttime descent toward Martha's Vineyard last year, losing control of his high-performance aircraft and spiraling into the Atlantic Ocean, the National Transportation Safety Board said yesterday.

The plane likely crashed because of Kennedy's "failure to maintain control of the airplane during a descent over water at night, which was a result of spatial disorientation," the board said. "Factors in the accident were haze, and the dark night."

The crash killed Kennedy, his wife, Carolyn Bessette Kennedy, and her sister Lauren Bessette as they flew from Caldwell, N.J., to the Vineyard on July 16, 1999, with Kennedy as the only pilot on the single-engine Piper Saratoga.

The sensitivity of the high-profile crash investigation was evident in the way the NTSB released the report, posting it on the agency's Web site without a formal meeting or vote by the five-person board. The report was mostly raw facts with little analysis, and no recommendations to prevent similar crashes--the treatment that other, less-noticed general aviation accidents usually receive.

A spokesman for Sen. Edward M. Kennedy (D-Mass.) said the family would have no comment on the NTSB report.

The report generally confirmed the thinking of most pilots and other aviation professionals at the time--that Kennedy had flown into a "black hole" with no horizon or lights from stars to help him keep his orientation. In that situation, pilots must slavishly trust their instruments because they will likely have illusions that make them unable to accurately determine how the plane is moving.

The report did add considerable detail about the hours leading up to the Kennedy takeoff, including weather forecasts that may have been misleading. His flight instructor also said he had offered to fly with Kennedy that night, but Kennedy replied "he wanted to do it alone."

"He [the instructor] also stated that he would not have felt comfortable with the accident pilot conducting night flight operations on a route similar to the one flown on, and in weather conditions similar to those that existed on the night of the accident," the report said.

The report painted Kennedy as a low-time pilot, with only 310 flight hours. But it said he was careful, methodical and seriously interested in becoming a better pilot. Investigators also determined that he had taken the same general route from New Jersey airports to the Martha's Vineyard/Hyannis, Mass., area 17 times in the 15 months before his death, with at least five of those flights at night.

Kennedy needed clear weather conditions the night of the flight. He was not under the control of air traffic controllers, and he had not yet received a rating that would have allowed him to fly only on instruments into conditions of poor visibility.

However, Kennedy had passed his written instrument examination on March 12, 1999, with a 78 percent score and had satisfactorily completed 12 of the 25 lesson plans that would allow him to be certified for instrument flying. His instructor said Kennedy's basic instrument flying skills were "excellent," but he "stated that the pilot had trouble managing multiple tasks while flying, which he felt was normal for the pilot's level of experience."

Kennedy also had sustained a foot injury in a hang glider accident and was wearing a cast on the night of the flight. The instructor said that on a flight June 25, three weeks before the crash, Kennedy performed a landing but needed help manipulating the rudder pedals.

The report said that most of the aircraft was recovered and no mechanical defects had been found. The instruments that Kennedy would have needed to fly the aircraft on a dark, hazy night showed no evidence of pre-crash failure.

However, there was an indication that Kennedy had turned off the autopilot before the crash. The autopilot contains an "altitude hold" feature that could have righted the plane and leveled it out automatically if Kennedy had used it.

The report said Kennedy "or someone using his user code" made two weather requests from Weather Service International's Web site, one for a radar image and the other for a "briefing" for his planned route. The reports were generally for clear skies and visibility of four to six miles in haze.

However, Kennedy did not access the National Weather Service Forecasts and Surface Weather Observations Web site. That site indicated somewhat worse conditions off the coast, with haze extending 7,000 to 8,000 feet, higher than his planned 6,000-foot cruise level.

Another pilot said, however, that he called a Federal Aviation Administration flight service office before leaving on the same route and "was told quite emphatically: No adverse conditions. Have a great weekend."

However, he said he later encountered poor conditions, saying, "I had no visual reference of any kind, yet was free of any clouds or fog."

The movements of Kennedy's plane appear to be similar to someone going through spatial disorientation. The plane made several turns, alternately climbing and descending. In the end, the plane went into an increasingly tight turn, its speed increasing and its nose turning down.

The plane's rate of descent reached an estimated 4,700 feet per minute before the aircraft hit the water, nose-down.