25 years later: Why did John F. Kennedy Jr.'s plane crash on the way to Martha's Vineyard?

It has been a quarter of a century since a six-seat airplane crashed into the Atlantic Ocean about 7.5 miles southwest of Martha's Vineyard, killing 38-year-old John F. Kennedy Jr., his wife Carolyn Bessette Kennedy, 33, and her sister, Lauren Bessette, 34.

The power couple was going to drop off Lauren Bessette at the Vineyard before continuing on to Hyannis to attend the wedding of JFK Jr.'s cousin, Rory Kennedy, to Mark Bailey at the Kennedy compound.

The search for JFK Jr. and his passengers covered 9,000 square miles and involved 10 Coast Guard vessels, two National Oceanic and Atmospheric Administration vessels, a Navy salvage vessel, two Air National Guard aircraft, two Coast Guard helicopters and a Coast Guard Falcon jet.

Read the full story: The deaths that broke Cape Cod's heart: Remembering the 1999 Kennedy plane crash

Why did JFK Jr.'s plane crash?

In the end, the National Transportation Safety Board found that JFK Jr. had become disoriented during his final approach to Martha's Vineyard after leaving the mainland coastal route he'd followed up to Point Judith on Narragansett Bay and striking out across open water.

According to the board's 28-page aviation accident final report, issued the following July, the probable cause July 16, 1999, was "the pilot's failure to maintain control of the airplane during a descent over water at night, which was a result of spatial disorientation."

Factors in the accident, the report found, "were haze, and the dark night." JFK Jr. was not yet rated to fly on instruments alone and he'd also had limited solo night-time flying experience, according to the report. The investigators found no mechanical failures and no indications of in-flight breakup or fire.

What is spatial disorientation in flying?

Spatial disorientation is the inability to determine body position or relative motion in the absence of visual cues. It is a factor in 5% to 10% of aviation accidents, with 90% of them ending with fatality, according to the Federal Aviation Administration.

While the weather that night required only that pilots be certified in visual flight rules — that is, flying with the visible horizon and at least three miles of visibility — other pilots who'd also flown in the Nantucket and Martha's Vineyard area that evening reported hazy conditions over the water and the islands.

One pilot, referenced in the report, said that visibility was unrestricted at 14,000 feet, but as he began his descent toward Nantucket over Martha's Vineyard, he looked down and, "…there was nothing to see. There was no horizon and no light...I turned left toward Martha's Vineyard to see if it was visible but could see no lights of any kind nor any evidence of the island…I thought the island might (have) suffered a power failure."

According to radar analysis, the plane had begun to descend from an altitude of 2,200 feet on approach to the Vineyard that evening, then rose in a turn toward the island, banked to the right and plunged into the ocean at a rate of 4,700 to 5,000 feet per minute, the Times reported. It disappeared from radar around 9:40 p.m.

Where are Carolyn Bessette Kennedy, Lauren Bessette and JFK Jr. buried?

Small debris from the plane was recovered July 19, 1999, three days after the crash. The deceased, and larger pieces of the wreckage, were recovered from 116 feet of depth July 21.

July 22, 1999, JFK Jr., his wife of nearly three years and his sister-in-law were buried at sea from the USS Briscoe.

The junior Kennedy's cousin, whose wedding he and his wife had been going to, quietly married two-and-a-half weeks later, in a small private ceremony in Greece.

Heather McCarron can be reached at hmccarron@capecodonline.com.

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This article originally appeared on Cape Cod Times: Why did JFK Jr.'s plane crash? What we know about the 1999 tragedy.