OTIS AIR NATIONAL GUARD BASE, Mass. (Reuters) - John F. Kennedy Jr. may have lost control of his plane in the final moments before its terrifying plunge into the sea, according to new data released by officials on Tuesday. National Transportation Safety Board officials said Kennedy‘s plane made a series of rapid turns and changes in altitude during its final moments on approach to Martha‘s Vineyard airport, which aviation experts said clearly indicated he lost control of the aircraft. "We call that a graveyard spiral," said Ernie Carnahan, a flight instructor of 18 years‘ experience from Ft. Pierce, Florida. "He may have lost his instrument lights. We may never know, but the bottom line is, he lost control." Kennedy‘s Piper Saratoga was about 30 km from the airport at about 700 metres when it started a turn to the right and climbed to 800 metres, where it remained for one minute heading southeast. About one minute before the last radar reading, which was taken when the plane was at 335 metres, it turned back toward the east, descending normally at about 215 metres per minute. But 30 seconds into that turn it started another right turn, then "entered a rapid rate of descent" of "possibly greater than 1,500 metres per minute," Pearce said. Kennedy, the only surviving son of slain President John F. Kennedy, his wife, Carolyn Bessette Kennedy, and her sister Lauren Bessette, all perished. Divers searching the waters off Martha‘s Vineyard on Tuesday for wreckage honed in on two particular target sites within a few hundred yards (metres) of where radar information indicates the plane hit the water. Several other possible wreckage sites were eliminated on Tuesday, Larrabee said, although officials said another seven or eight sites had been identified and would need to be investigated. Divers were working in difficult conditions, including 11 degrees Celsius water at depths of 35 metres, with only about 2.4 metres of visibility. Two National Oceanographic and Atmospheric Administration (NOAA) ships, the Rude (pronounced Rudy) and the Whiting, use sonar to detect debris on the ocean floor. Searchers are using three search ships, a patrol boat and the U.S. Navy salvage ship USS Grasp. Police on Martha‘s Vineyard said among the debris that washed up on Saturday were documents identifying the aircraft. The plane crashed around 9:40 p.m. EDT on Friday (0140 GMT Saturday) on a flight from New Jersey. Breaking their silence on the loss of Kennedy, the family said on Monday they were stricken with "unspeakable grief and sadness."