On a sticky summer evening 20 years ago, Trans World Airlines Flight 800, carrying 230 people, climbed south from Kennedy Airport, then turned east for Europe.

The Boeing 747-100’s passengers, who included a Center Moriches dentist and his wife celebrating their sixth wedding anniversary, 16 members of a Pennsylvania high school French club, and parents taking children on their first European adventures, were eagerly expectant of a trip to Paris.

But the jet never made it to Europe. At 8:31 p.m., 12 minutes into the flight on July 17, 1996, an explosion split the plane in two about 15,000 feet over the Atlantic, some 8 miles south of the beaches that flank the East Moriches inlet.

So were the first moments of a tragedy that still brings hard memories and persistent questions two decades after funerals for the victims.

The disaster prompted the largest investigation of an aviation crash in U.S. history, according to officials at the National Transportation Safety Board, and galvanized Long Island as few events have.

Randy Penny was helping to host about a half-dozen dinner guests on his father’s lobster boat near the dock at Great Gun Beach when something caught his eye.

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“It was perfectly white, like a light bulb — no smoke, nothing,” said Penny, 52, a boatyard worker who lives in Moriches.

“But that turned into a forest fire with orange and yellow, a big fire and two rolling things that went down behind the dunes,” he said. “We ran up on the dunes and could see smoke on the water. We thought two planes had collided.”

Federal aviation officials have said a spark from fuel gauge wiring inside the center wing fuel tank of Flight 800 caused a huge explosion.

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Since then, the Federal Aviation Administration has issued some 375 directives aimed at preventing sparks from endangering the fuel tanks of commercial airliners.

But airlines generally will not be required until 2017 to fully comply with rules designed to prevent similar crashes — rules the FAA announced in 2012.

Agency officials say those rules — requiring large commercial airliners to be fitted with equipment to block oxygen from reaching combustible vapors trapped inside center fuel tanks — likely would have spared the lives of the passengers and crew.

“Four fatal airplane accidents have been caused by fuel tank explosions since 1989,” the FAA said in a 2005 notice of proposed rulemaking. “A system designed to reduce the likelihood of a fuel tank fire, or mitigate the effects of a fire should one occur, would have prevented these four fuel tank explosions.”

Victims’ family members continue to be haunted by the tragedy.

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Richard Penzer, of Lawrence, still wonders what might have been had a sibling, perpetually late, missed the flight. His sister, Judy Penzer, one of the 218 passengers and 12 crew members on the flight, had begun collaborating with him on real estate projects in deteriorated sections of Pittsburgh in the mid-1990s.

An avid painter, she adorned many of the buildings Richard had purchased, including an 11-story mural on the wall of a former department store that depicted Pittsburgh sports heroes — Roberto Clemente, Mario Lemieux, “Mean” Joe Greene and others.

“Has it been 20 years already? It seems like yesterday, the years pass so quickly,” said Penzer, 58, of Lawrence, who also worked with Judy Penzer on charitable projects in Brooklyn. “She was doing what she believed in, her painting and working on liberal causes.”

“It was especially hard on my mother,” Penzer said of Judy Penzer’s death. “Right away, she deteriorated immediately. And it would have killed my father had he been alive.”

Richard Penzer’s daughter, Ariel Penzer, 25, herself a fashion artist, was only 5 years old when her aunt was killed.

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But she grew up surrounded by the artwork that Judy Penzer, 49, painted, which now hang from almost every square foot of available wall space in her family’s Lawrence home, and which give the house the feel of an art gallery.

“I don’t really remember her much because I was very young when she died,” Ariel said. “But I really get a feeling for her spirit through her paintings.”

Within minutes of the plane plunging into the Atlantic, scores of Long Island boaters were speeding toward the wreckage in a vain rescue effort. And for weeks afterward, East End townspeople endured the almost constant presence of emergency crews and television trucks. Because some claimed they had seen a missile, Federal Bureau of Investigation agents investigated whether terrorism was involved, knocking on doors and searching for witnesses.

And ordinary Long Islanders came to realize their lives had not gone untouched.

Rita DiBella, 57, of Mastic, had been singing with a country band that night when the group was told to end early because of an emergency. Days later, she learned her beloved family dentist, Eric Holst, 32, and his wife, Virginia, 31, were among the dead. DiBella still visits a memorial to the victims during walks at Smith Point County Park, a few miles from her home.

Ed Probst, 60, an East Moriches homebuilder who owns a 29-foot fishing boat, was among those who had tried to help that night.

Within a half-hour of the crash, he had gathered two friends in a boat and sped south from Moriches Inlet, eventually picking his way through the plane’s debris before coming across the bodies of several victims.

“It shook our town up,” he said. “For weeks, it was all reporters and funerals.”

Both the NTSB and the Federal Bureau of Investigation initially were in charge of the investigation. Federal agents, who interviewed at least 736 eyewitnesses, initially speculated that the plane may have been brought down by a bomb or a missile.

At least 258 of the eyewitnesses, including an Air Force National Guard helicopter pilot who had been flying a training mission within sight of the explosion, said they saw a streak of light approach the plane before the 8:31 p.m. explosion.

But after retrieving some 9,600 pieces of the wreckage from the Atlantic and reassembling much of the plane in a former Grumman Aircraft hangar in Calverton, the FBI in November 2007 said there was no evidence of criminal activity.

Four years later, the NTSB concluded that the explosion was likely caused by a spark that ignited aviation fuel fumes in a nearly empty fuel tank in the plane’s belly.

Although the word “missile” is mentioned more than 190 times in the NTSB’s Aug. 23, 2000, “Aircraft Accident Report” on the disaster, it explicitly rejected the possibility that a missile was involved.

NTSB officials said the witnesses likely mistook trails of burning jet fuel for an ascending rocket, and that there were no signs of the damage pattern a high-velocity missile attack would produce.

“The National Transportation Safety Board determines that the probable cause of the TWA Flight 800 accident was an explosion of the center wing fuel tank, resulting from ignition of the flammable fuel/air mixture in the tank,” the NTSB said in the executive summary of Flight 800’s 417-page accident report.

The plane is now permanently housed at the NTSB Training Center in Ashburn, Virginia — 12 miles northwest of Washington, D.C. — where the jet is used as a teaching tool for the agency’s crash investigators. The NTSB also invites law enforcement, academics, military and others interested parties to examine the three-story reconstruction to learn how to spot clues during crash investigations, and to avoid future disasters.

The NTSB has allowed families of the victims and others with emotional ties to the disaster to visit the reconstruction, and even to lay flowers on the aircraft’s seats. One recent visitor was a former TWA pilot who had flown the doomed craft from Athens to Kennedy Airport , its last journey before the crash.

The crumpled 100-foot section of the plane’s midsection still bears its red “Trans World” logo, shredded but recognizable on the craft’s white fuselage. Stunningly massive, the jet is kept in a cavernous room just off the training center’s lobby.

Paul Schuda, training center director, used a laser pointer to show where the explosion originated near the intersection of the aircraft’s right wing and fuselage.

“The outward bowing there, and the upward bowing indicates the force of the explosion in the center tank,” Schuda said.

Aircraft fuel tank explosions are a rare but real danger.

There have been 18 such explosions on commercial transport aircraft since the Federal Aviation Administration began counting them in 1959, including a 1990 blast that killed eight of 120 people aboard a Philippine Airlines Boeing 737 as the jet was taxiing at an airport in Manila.

A statistical evaluation by the FAA predicted that unless something was done to address the risk there could be nine more fuel tank explosions on commercial aircraft over the next 50 years.

The FAA initially rejected as too expensive an NTSB recommendation that commercial airliners be fitted with systems that rendered explosive vapors in fuel tanks inert. The inerting systems were projected to cost from $140,000 to $225,000 per plane.

But the FAA reversed itself in 2005. A “Notice of proposed rulemaking,” issued that year read “fuel tank inerting, originally thought to be prohibitively expensive, can now be accomplished in a reasonably cost-effective fashion and protect the public from future calamities which, we have concluded, are otherwise virtually certain to occur.”

After the TWA Flight 800 crash, 16 years passed before the FAA imposed new rules, in 2012, requiring airlines to take steps to render jet fuel vapors incapable of exploding. Airlines may do so either by injecting nitrogen or some other inert gas to displace oxygen in partially empty fuel tanks, or by using polyurethane foam in the tank to inhibit ignition of the vapors.

The new FAA rules gave airlines until the end of 2014 to bring at least half of their fleet into compliance, and until Dec. 26, 2017, to be within full compliance.

The rule applied to all new aircraft, and required the airlines to retrofit the nearly 3,300 wide-body aircraft that did not already have inerting systems.

But the rule exempted several models of planes built before 1992 and still flying, including the McDonnell Douglas DC-10, the Boeing 727 and the Lockheed L-1011. According to an advisory, the FAA exempted those models because “their advanced age and small numbers would likely make compliance economically impractical.”

Still, FAA officials say they have taken reasonable steps to eliminate the risk of fuel tank explosions.

“Flying on a U.S. airline is incredibly safe and the fuel tanks on commercial airplanes are far safer today than they were 20 years ago,” according to a statement released by FAA spokeswoman Alison Duquette.

A spokesman for Airlines for America, an organization representing the airlines industry, said the time allotted for implementing the tougher fuel tank regulations was fair, given the challenge of renovating thousands of planes with new equipment.

“The implementation schedule adopted by the FAA acknowledged the complex manufacturing, engineering, and technical challenges of installing inerting systems on aircraft, while also taking into account the low risk of a similar incident occurring due to the airline safety measures already in place,” the spokesman said. But that has not satisfied the relatives of some victims.

John Holst Jr., 53, of North Babylon, brother of victim Eric Holst, said aviation officials have been too deferential to the airline industry. He pointed out that military aircraft have used gas inerting systems for decades, including the F-16 fighter, which injects inert halon 1301 that is used in some fire extinguishers.

“They are still allowing airplanes to fly without inerting systems 20 years later,” he said. “Are they waiting for another plane to blow up?”

DiBella, Eric Holst’s former patient, works as an administrator at Senix Marina in Center Moriches, where eyewitness Randy Penny also works, and from which several boats were launched the night of the tragedy in a futile rescue attempt.

She said despite the passage of time, memories of the tragedy still haunt Long Island’s East End communities.

“It happened right here,” she said. “I knew a lot of people who went out to rescue them.

“But it turned out to be a recovery — there was no one for them to rescue,” she said. “And when you knew someone who perished in something as big as Flight 800, it really hits home.’’