WASHINGTON – Current and former federal officials who played key roles in the investigation of one of the nation’s worst aviation disasters said Tuesday they stand by their conclusion that the explosion of TWA flight 800 was caused by overheated fuel tank vapors, and not a bomb or missile.

The officials spoke to reporters at a briefing on the National Transportation Safety Board’s four-year investigation following the explosion and crash of the Boeing 747 off Long Island, N.Y., on July 17, 1996, killing all 230 people on board. The board took the usual step of organizing the briefing on an investigation that has been closed for years. That’s in response to a new documentary film set to air this month on the 17th anniversary of the tragedy. The film says new evidence points to the often-discounted theory that a missile strike may have downed the jumbo jet.

The officials who spoke at the briefing dismissed allegations of a cover-up, saying that the evidence points strongly to the board’s conclusion that overheated gases in the plane’s near-empty fuel tank caused the tank to explode. The gases were most likely ignited by a spark from damaged wiring in a fuel measuring system.

Joseph Kolly, the current director of the board’s Office of Research and Engineering, was the chief fire and explosives investigator on the flight 800 investigation. He said he is “absolutely” certain the fuel tank was the cause.

“We went to the ‘nth’ degree, and then some,” he said, laying out the different facets of the investigation. In their search for clues, investigators tested shoulder-fired missiles to see if they would show up on radar and used another 747 to replicate the overheating of fuel tank vapors, among other tests.

“I am upset about bringing this back up, for the sake of the people who lost folks in the accident,” Kolly said. “It’s not good.”