The helicopter that would become his obsession flew by Frank Owens that rainy spring day in 1993.

It was a military helicopter from the elite Marine Corps unit whose polished aircraft are a presidential fixture, lifting off from the South Lawn and landing at Camp David. The helicopter's pilot sometimes had ferried George Bush; President Clinton himself had been aboard the machine two months earlier.

But such knowledge lay in the future. On that day, Owens, an amateur archaeologist, was fixed on digging for evidence of Union encampments in the Southern Maryland countryside.

Owens saw the helicopter fly past his digging site on the Potomac River's east bank but thought little of it. When the rain worsened minutes later, he packed his equipment and headed home along the winding back roads of Charles County's remote Nanjemoy district.

In thin woods off to his right, he spotted the crumpled tail section of a downed helicopter -- the shattered remains of Marine Helicopter Squadron One's flight Nighthawk 18. Jet fuel still was pouring from ruptured tanks as Owens rushed up. All four crew members lay dead or dying.

He grasped the hand of the prone Maj. William S. Barkley Jr., a command pilot and the only Marine not engulfed by the twisted wreckage. Owens later would describe how he offered a silent prayer and how, in that instant, his life changed.

"As I released his hand, I felt a wonderful peace and calmness," Owens wrote to Barkley's widow, Sylvia. "The inspiration and drive behind me is Bill."

So began Owens's fixation on the fatal flight, and his sprawling amateur investigation of what he calls a military coverup. Along the way, he has scoured the crash site for helicopter fragments, studied graphic autopsy photos, driven to West Virginia in search of clues and prowled a remote peninsula, seeking evidence of secret military activity.

His home office a few miles south of Waldorf is consecrated to the accident. Tucked into one corner is a detailed diorama of the crash scene, complete with model trees and a model helicopter. Pinned on a bulletin board are snapshots of Barkley, grinning with family or peering from a helicopter cockpit as Bush bounds from the craft. Wall charts display the flight path and other key documents, while video and audio tapes lie loosely catalogued, awaiting their umpteenth review.

The material, Owens contends, sketches a netherworld of malign scientists, top-secret "black" projects and needless deaths. He propounds a detailed scenario for the downing of Nighthawk 18: The helicopter was shot down, he says, by a Star Wars-type energy beam fired from a secret military installation engaged in experimental testing of electromagnetic pulses.

It's a seductive theory for the era that spawns rumors about conspiracy in the death of White House aide Vincent Foster rumors and conjecture that the federal government blew up one of its own buildings in Oklahoma. In such an era, it's not surprising that radio host G. Gordon Liddy, in a broadcast this year, would judge Owens's evidence to be "explosive."

With Star Wars research a declared fact and secret government installations a lurking reality, who's to rule out such a scenario?

The Marine Corps, which investigated the disaster, considers the scenario unfounded. The service officially has blamed mechanical failure brought on by faulty maintenance for the crash.

But Owens decries that conclusion as corrupted by the service's blind self-interest in standing by its own findings, and possibly at the directive of unseen higher-ups.

"They are not going to tell me," said Owens, an Army veteran who works in a grocery store. "But I feel there's something here the American people should know. If I were to tell you this on the telephone, you'd think I was some kind of nut case. It's fantastic, you have to see it to believe it."

At least some of the victims' relatives back Owens.

"If it hadn't of been for Frank, we'd have no clue," Matt Barkley, younger brother of the dead pilot, said in a telephone interview from his Hickory, N.C., home. "They've tried to frame him as this wacko-type guy, but they've never met anyone as good as he is."

Matt Barkley, a Citadel graduate, is a lifelong resident of small-town North Carolina and a recipient of the bedrock patriotism learned there. Yet, two years of fruitless entreaty have dimmed his once-unalloyed admiration for the Marine Corps. He's not sure he buys into Owens's theory of experimental testing gone awry. But he does believe important questions about the deaths of his brother and three other Marines need answering.

Some relatives of the other crash victims are also unsettled and have joined in a $48 million lawsuit that accuses the manufacturer of the helicopter and its engine of negligence. Stacie Reynolds, widow of Barkley's co-pilot, Capt. Scott Reynolds, said in an interview that she hoped the lawsuit would bring forth more facts.

The Marines' explanation of the crash is this: The VH60N Black Hawk had been taken out earlier that day by another pilot, who noticed the engine over-speeding during autorotation -- a routine maneuver that has the pilot temporarily cut power to the rotors. He returned to the Marine Corps Air Field at Quantico and reported the anomaly.

Barkley, who was not slated to fly for several more hours, volunteered to take the craft up and check the problem. His qualifications were excellent: He had more than 5,000 hours of helicopter flight time and was one of the select few within the select squadron who was authorized to fly the president.

He and three others planned to take the craft across the river, test the autorotation over Southern Maryland's patchwork of fields and woods and fly on to Bolling Air Base by the Anacostia River, where another helicopter needed attention.

Minutes after liftoff, they had their last recorded conversation with the Quantico control tower, a routine transmission: "Nighthawk 18, east bank cleared, like to remain your frequency."

Minutes after that, with no recorded distress call, the four lay dead, their bodies and their craft shattered near Jacksontown Road.

Two military inquiries investigated the disaster and reached the same conclusion: Nighthawk 18 crashed because a mechanic misinstalled a pin connecting two tubes that link the engine to the transmission. The fault manifested itself only under the stress of a briskly executed autorotation and unexpectedly shut down both engines.

Owens said he first became suspicious when the Marines reported that Barkley's body remained in the wrecked helicopter. Owens is certain the pilot lay clear of the machine. "Why would they lie about that?" he said.

As he probed further, Owens said, he discovered a series of seemingly odd facts. Some of them, and the inferences he drew, include:

The crewmen suffered burns, even though there was no fire at the crash. They bled little, despite massive injuries.

Owens believes that the men were burned fatally by powerful microwaves that, in a process similar to electrocution, cauterized their blood and prevented bleeding.

The Pentagon in the 1980s conducted secret tests at Woodbridge of devices emitting powerful electromagnetic pulses. The command was forced to halt those tests amid concern about aviation safety but has another laboratory across the Potomac River in Maryland, six miles from Nighthawk 18's crash site.

Owens believes that scientists at that second site conducted clandestine EMP experiments that downed Nighthawk 18.

The helicopter's fuselage was shattered, yet its rotors suffered little damage.

Owens believes that the helicopter exploded, but not from conventional charges, given the lack of fire. It sustained its damage in the air, not upon impact, or the rotors would have been be mangled.

Owens may have accurately winnowed out some odd facts surrounding the crash, but his inferences are not supported, according to independent aviation and medical specialists consulted by The Washington Post and police and rescue personnel summoned to the crash scene.

Take the location of Barkley's body. Owens recalls the pilot clear of the craft. Yet in interviews with The Post, rescue workers and police described, variously, nobody visible in the tangled wreckage, one body visible but still strapped in, or one thrown well clear of the craft.

Or take the seemingly mysterious burns where there was no fire and the lack of bleeding. At The Post's request, Virginia's chief medical examiner inspected the military autopsy photographs of the bodies.

"If somebody just sent me these pictures and asked, "What happened to these people?' I'd say aircraft,' " said the examiner, Marcella Fierro. "Their injuries are all typical. . . . Is this an uncommon finding? No. Do you need a fire? No."

And what of the possibility of directed-energy weapons testing in Southern Maryland? Here, Owens extrapolates from recorded fact.

The Pentagon, under pressure of a lawsuit, halted EMP testing at Woodbridge in the late 1980s. Aviation authorities feared the testing might cause a crash by interfering with the electronic circuitry that controls modern aircraft. The Blossom Point Field Testing Facility, six miles from the crash site in Charles County, was under control of the same command that carried out the Woodbridge testing.

Owens asserts that the command simply moved its pulsers from Woodbridge to Blossom Point. But David Davison, spokesman for the U.S. Army Research Laboratory, said in a telephone interview that the pulsers were moved to the White Sands Missile Range in New Mexico and that Blossom Point never made EMP tests. A device at Blossom Point that Owens and a former Army laboratory employee call a possible EMP machine does emit electromagnetic waves, but at low levels and continuously, rather than in pulses, Davison said.

The devastation of the helicopter and the relatively intact rotors raised no eyebrows among four independent aviation accident specialists contacted by The Post. Several emphasized that mechanical failure along the lines cited by the Marines would cause a rapidly escalating sequence of failure that would turn the Black Hawk into a five-ton falling rock.

All of the independent findings are no surprise to the Marines, who reached their conclusions in a report of more than 380 pages, a document that drew upon the expertise of aviators, engineers, lawyers and pathologists. Several independent helicopter specialists who reviewed the report at The Post's request judged its conclusions to be sound and the investigation to have been thorough.

The Marines, who have answered Owens's questions for more than two years, now simply wish that he would desist.

"The same questions go over and over and over again. It's almost vendetta-like," said Robert C. Jenks, a Marine Corps spokesman. "Frank Owens needs to move on."

He is.

In his study one recent day, Owens displayed a photocopy of a postcard said to have been sent from a tropical island to the FBI. The signature appeared to be that of a person presumed murdered, yet whose body was never found. . . . CAPTION:Owens built a model of the crash site, with labels marking were the bodies of the four victims were found. He believes the helecopter was shot down by a Star Wars-type energy beam in a secret military experiment. CAPTION: Maj. William S. Barkley Jr., above, a command pilot, died in the crash of Nighthawk 18. Left, Frank Owens, who has devoted himself to an amateur investigation, holds debris he has gathered from the helicopter crash site.