UPDATE : Clues Sparse in Train’s Derailment : FBI has questioned hundreds about sabotage in desert that killed one, hurt 78. Right-wing manifesto was found at scene.

William Marks was a prime suspect when he was hooked up to a polygraph machine and interrogated by federal agents in connection with the deadly derailment of an Amtrak train near this desert community.

During hours of questioning by agents who “stared into my eyes the whole time,” Marks was told that someone had implicated him in the sabotage of a trestle that sent the Sunset Limited crashing into a ravine on Oct. 9, killing one person and injuring 78.

He also was asked about his gun collection, night-vision rifle scope and whether he knew any locals who hated the federal government. And, they wanted to know, “Did you do it?”

With wires attached to his body to monitor his breathing, heartbeat, blood pressure and perspiration, the 33-year-old welder answered, “No.”

“I was cleared,” Marks recalled recently, with a sigh of relief, at his home near here. “But, man, it was demeaning. I felt about that tall,” he said, holding his thumb and forefinger an inch apart.

The polygraph test was another dead-end for FBI investigators in this isolated area 55 miles southwest of Phoenix, where coyotes scratch out an existence along with farmers, ranchers, day laborers and loners who do not welcome law enforcement’s intrusion.

Over the last five weeks, federal agents have talked with hundreds of residents here--from cowboys to video store clerks--in search of clues that could tie a person or group to evidence gathered near where the 12-car train crashed in the middle of the night.

Investigators found a “manifesto” near the crash site signed by “Sons of Gestapo.” The one-page note mentioned the government sieges near Waco, Tex., and at Ruby Ridge, Ida.--both rallying cries for far-right extremists.

There were reports of a person wearing a cowboy hat walking away from the wreckage. Rails at the crash site were wedged apart and connected with electrical cord to keep the circuitry intact, and train signals green along the track.

But there were apparently no witnesses, and even a reward of $100,000 has thus far failed to produce arrests in the murder case.

In a federal effort that is only exceeded by the investigation into the bombing of the Oklahoma City federal building in April, FBI agents have interviewed more than 1,000 people across the country and responded to 500 hot-line calls for crucial leads in the case.

While authorities are leaning toward a theory that the derailment may have been the work of a disgruntled railroad employee familiar with railroad operations and the local geography, they have not ruled out the possibility that political terrorists are to blame.

“At this time, there are many potential suspects we are looking at--not all of whom live in Arizona--and we haven’t eliminated any scenario,” said FBI spokesman Jack Callahan. “I can’t tell you an arrest is imminent.”

Last week, FBI Director Louis J. Freeh said that the agency has “a couple of very strong leads that we’re working on. There are a couple of aspects to the case that we are, at this point, mildly optimistic about.”

But in what locals see as a sign that the investigation is starting to wane, authorities have returned to Hyder and nearby communities of Arlington, Buckeye and Gila Bend for additional interviews.

Sherry Smith, a 23-year-old video store manager, is among many who have been interviewed several times.

“They asked me if I knew anyone who is racist or prejudiced, and I said that about sums up the whole community,” she said. “And they asked if I knew anyone who drives a four-wheel- drive vehicle or wears a cowboy hat. That’s everybody too.”

Authorities showed initial interest in three hunters who were camped by a dry wash about six miles from the crash site.

“I told the FBI it was OK to search my place,” one of those hunters, who asked not to be identified, recalled. “And that was the start of a . . . roller-coaster ride that lasted a week and a half.

“But they were extremely courteous and careful, and by no means trashed my house,” he added while nursing a beer at the Desert Rose bar. “My wife didn’t even know they’d gone through her things when she got home from work, and, hey, that makes you kind of proud of the FBI.”

But as he watched a coyote dart across his yard, Marks said: “If they’re talking to me about it, they don’t have a clue who did it.”