Three detectives pulled up to a rugged piece of land here, a 220-mile drive from their offices in suburban Maryland.

To their right was a small white house. Beyond that, a rutted dirt road extended up a steep slope, bending through towering trees.

The detectives walked up a 100-foot driveway and knocked. Nikki Arrington, 30, answered, telling them she just started renting the place and it would be fine for them to look around. She pointed to the dirt road. “There’s a family graveyard up there on the hill,” she said.

The trip, six months ago, had come after significant progress in their efforts to solve an indelible, heartbreaking mystery — the 1975 disappearance of two Montgomery County girls, Sheila Lyon, 12, and her sister, Katherine, 10. The girls vanished after walking to a Wheaton shopping mall to look at Easter decorations and eat pizza.

The detectives had zeroed in on a man who had family connections to the land — a convicted sex offender who had gone so far as to tell detectives he’d left that mall with the Lyon sisters, according to court papers.

Timeline and map of events in the search for the Lyon sisters

To search the land, though, the detectives and their colleagues in Virginia have had to call in FBI specialists trained to dig up dirt, run it through sifters, and look for clues that could be smaller than the eraser on a pencil.

“This is a tough case, trying to piece together things from 40 years ago,” said Randy Krantz, the top prosecutor in Bedford County, the area where the digs are taking place.

Over the years, a series of investigators have worked the case, many going over the same aged records stored in files.

In May 2013, Detective Dave Davis had just joined the investigation when he and others set fresh eyes on a report in those files: An interview detectives had with a man named Lloyd Welch in 1975, just a week after the Lyon sisters disappeared. Welch hadn’t been considered a suspect back then, but the more present-day detectives looked at him, the more interested they became.

Heightening their suspicions was what he had done since he left Maryland.

In 1992, in South Carolina, a 10-year-girl who had been staying with him watched a horror movie, became frightened and got into bed with Welch. She awoke to find him molesting her. Welch pleaded guilty in 1994 and was sentenced to 18 months in jail. Three years later, in Delaware, Welch started playing pornographic movies in the presence of another 10-year-old girl. He abused that girl over more than a week, pleaded guilty and was serving a lengthy sentence in Delaware.

By early 2014, two more detectives were on the case: Katie Leggett and Mark Janney. She had spent 12 years investigating child abuse. He had spent much of his career putting together complicated drug conspiracy cases.

Both had young daughters of their own — something that became part of their conversations about the case and made them think constantly about the family of Sheila and Katherine. The girls’ parents are still alive, still looking for answers, as are their brothers, one of whom had gone on to become a cop for Montgomery County.

Janney said he has tried to imagine what their life has been like, but the anguish is beyond description. “I can’t even fathom it,” said Janney.

He, Leggett and Davis — along with their sergeant, Chris Homrock — agreed to speak generally about the case. Citing an ongoing grand jury investigation, they declined to talk about any evidence collected or interviews conducted.

The day the trio first arrived at the land and looked at the dirt road, they realized immediately it was too steep for their unmarked sedan. They headed up on foot, passing through dense woods on either side, and came to a clearing. At first the headstones were difficult to see — small and unmarked — but there were more than 30 of them. Only one marker could be read, that of a woman who died in 1906.

The detectives checked into a hotel that night, and the next day they paid a call to the Bedford County Sheriff’s Office, which quickly summoned Krantz, the top prosecutor. They brought him up to speed on their case.

Authorities wouldn’t detail the conversation, but a request for a search warrant filed in Bedford court outlines how detectives had begun to focus on the Virginia mountain.

Their interest in Lloyd Welch led to an interest in his relatives, including an uncle, Richard Welch, who lived just outside of the District, in Hyattsville. Richard Welch owned a small parcel of land in Bedford. Next to that parcel is 34-acres formerly owned by Richard Welch’s sister, according to the affidavits.

The detectives also learned that in the months after February 2014 — when police officials in Montgomery County announced that Lloyd Welch was a “person of interest” in the case — Richard Welch and his wife Patricia had made several trips to Bedford County, according to the affidavits. Richard and Patricia appeared “extremely interested” in whether any of their relatives had spoken with investigators, and if so, what they said. The detectives spoke with one Welch relative who said she remembered Lloyd Welch visiting his family’s property in the spring of 1975.

Krantz, the prosecutor, knew the area they were talking about — Taylors Mountain — having grown up nearby. It was one of the areas in the county his parents had told him to stay away from at night. Too much drinking, which could lead to too much fighting. But Krantz also knew another side as well, the part he saw as he was seeking donations for the volunteer rescue squad. Those who came to the door were friendly, and they gave what they could.

Krantz and the sheriff’s officials told the Montgomery detectives they would help. The detectives headed home for the weekend, returning the next week to find a large basement room at the sheriff’s department had been turned into a command center. There were about two dozen people, including county and state investigators, ready to search the mountain.

To members of the Welch family, the detectives efforts’ have amounted to an obsession — one that has led them into a form of tunnel vision.

“I tried to tell them I had nothing to do with this, and look where it got me. My family’s and my lives turned upside down,” Lloyd Welch wrote in a letter to The Washington Post from prison.

“They can dig that whole mountain down and they’re not going to find them, because they’re not there,” said Welch’s stepmother, Edna Welch, who lives in Tennessee.

Richard and Patricia Welch have declined to comment. But their daughter, Patricia Ann Welch thinks detectives are putting too much stock in how Lloyd Welch has implicated her father. According to court papers, Lloyd Welch said he last saw the girls with Richard Welch, and that his uncle was sexually assaulting one of the sisters.

“They need to back up and do their homework. Somewhere they went from Point A to Point Z,” she said. “My father is an innocent person.”

Statistics bear out how unusual it is to obtain murder convictions without finding a body, or traces of a body.

There have been 435 such trials known in U.S. history, according to statistics kept by Tad DiBiase, a former assistant U.S. attorney in the District, and the author of “No-Body Homicide Cases: A Practical Guide to Investigating, Prosecuting, and Winning Cases When the Victim is Missing.”

Prosecutors typically use some combination of witness testimony, confessions or DNA found in blood stains at the crime scene. In the Lyon sisters case, after 40 years, and with the searches being outside, finding physical clues is particularly challenging.

Indeed, should prosecutors try to bring such charges in the Lyon case — with no physical evidence — it would represent the longest known duration ever between a person’s disappearance and a trial, according DiBiase.

Still, he stressed that such an effort could be successful. Of those 435 trials he documented, nearly 90 percent ended in convictions. “It certainly can be done,” he said.

The detectives say they will keep looking — for evidence, for answers, for however they can tie it all together.

“I want justice for these girls,” said Leggett. “And I want an explanation for their family.”