The National Transportation Safety Board (NTSB) said the plane showed no signs of in-flight breakup or fire before it crashed July 16, killing Kennedy, his wife Caroline, and her sister Lauren.
The NTSB also said that 75 percent of the plane was recovered from the waters off Martha's Vineyard, Mass., including a primitive recording device. That instrument, however, was destroyed by the impact.
The safety board, in its statement, also said that Kennedy had received a weather forecast via the Internet for his flight from Fairfield, N.J., to the island off Cape Cod about 6:30 p.m. on July 16. The report, issued about two hours before takeoff, was for good visual-flight-rules conditions, with six to eight miles visibility.
No weather warnings were issued to pilots flying Kennedy's route, which took him over the Long Island Sound along the southern coast of Connecticut.
"However, pilots who had flown over Long Island Sound that evening reported after the accident that the in-flight visibility over the water was significantly reduced," the safety board's statement said. "Interviews of those pilots will continue."
After divers helped a recovery team on the USS Grapple pull up the wreckage, pieces of John Jr.'s plane were handed over to investigators in Newport, R.I., earlier this week.
Later, they were taken to a secure hangar at the Coast Guard Air Station on the grounds of the former Otis Air Force Base. There, the parts of the plane were laid out in a very rough reconstruction on a hangar floor.
Kennedy, the 38-year-old son of President Kennedy, his wife, Carolyn Bessette Kennedy, 33, and her 34-year-old sister Lauren Bessette, were flying in Kennedy's single-engine Piper Saratoga when it crashed about 7 and 1/2 miles southwest of the Vineyard.
The couple had intended to drop off Lauren Bessette on the island before flying to nearby Hyannis, Mass., for the wedding of Kennedy's cousin Rory, youngest child of the late Sen. Robert F. Kennedy.
After a five-day search, the bodies of the three victims were located in 116 feet of water, still strapped into their seats.
The propeller "indicated the presence of rotational damage," which would indicate it was still spinning when the plane crashed. Radar tapes show the plane plummeted into the water at a descent rate of perhaps greater than 5,000 feet per minute at about 9:40 p.m.
The engine was sent to Williamsport, Pa., for examination by its manufacturer, Textron Lycoming, while the propeller was transferred to a facility near Dayton, Ohio, for examination by Hartzell Propeller. Both inspections were conducted under supervision of the safety board, as is standard practice.
"No evidence wafound during the examinations of conditions that would have prevented either the engine or the propeller from operating," the statement said.
The board said the aircraft's Global Positioning System unit and selected cockpit instruments will be sent to its lab in Washington for examination. The radios, meanwhile, have been hand-carried to their manufacturer, Allied Signal, in Kansas.
The statement said investigators have learned that Kennedy had about 300 hours of flying experience, not including his time in the Saratoga, which he bought in April. Kennedy received his pilot's license in April 1998.
The NTSB has said it hopes to complete its investigation in six to nine months.