Shirley, N.Y. ? A little past 8:30 p.m., as the blistering July sun melted in the west, a jumbo jet carrying students, honeymooners, businessmen and others to Paris exploded in a fireball, raining carnage into the Atlantic Ocean off Long Island.

It had been only a dozen minutes since TWA Flight 800 took off from Kennedy Airport.

Initially, investigators were not sure whether the calamity that killed all 230 people aboard the flight on July 17, 1996, was caused by a bomb, a missile or mechanical failure. Just two days before the start of the Summer Olympics in Atlanta, it was difficult not to suspect foul play.

People still question the official conclusion, reached in 2000, that TWA 800 was destroyed by an explosion in the Boeing 747’s center fuel tank, likely caused by a spark from a wiring short-circuit.

The man who spent years leading the federal investigation has no such doubts.

Robert Francis, the former vice chairman of the National Transportation Safety Board, still gets missives urging him to come clean about terrorism theories or witness accounts of a streak of light heading toward the plane.

Sitting near a rose she placed in the sand at Smith Point beach in Fire Island, N.Y., in this file photo of July 26, 1996, a woman mourns the loss of some of her friends who were part of the TWA Flight 800 crew. A decade later, people still question the official explanation, reached in 2000, that TWA 800 was destroyed by a center fuel-tank explosion.

Now retired and living in McLean, Va., Francis said he never responds to “very insulting e-mails, asking me when am I going to tell the truth and asking why am I lying.”

Because Flight 800 disintegrated over the ocean, Francis said, there was ample time for conspiracy theories to flourish.

“There was no ability to make a determination quickly,” he said. “We were picking up the wreckage in 130 feet of water.”

Investigators eventually recovered 98 percent of the wreckage from the ocean floor and painstakingly rebuilt a large part of the airliner.

“We studied if it had been a missile, what would it have done to the fuselage? We had scientists in the desert shooting rockets into old fuselages,” he said in a recent telephone interview. “We didn’t find a single piece of wreckage that would have pointed to that kind of explosion. So I say to the missile theorists: ‘Show me something.'”

Another theory was that a missile from a Navy ship 200 miles away took down the plane, but Francis questioned why no one ever came forward. “You mean to tell me everybody kept their mouths shut?”

He said investigators also searched for evidence that a terrorist had launched a shoulder-fired missile from the nearby shore. “We never found any of that,” he said. “They searched every marina, every wharf.”

But why did so many people have doubts?

“It’s more acceptable to the public if somebody did this, rather than blame it on maintenance or design,” said Pete Field, a retired Marine Corps pilot who now works as an aviation crash consultant.