A sex offender named as a “person of interest” in the 1975 disappearance of two young sisters in Montgomery County has told detectives that he left a shopping mall with them the day they went missing and that he later saw his uncle sexually assaulting one of the girls, according to police affidavits recently unsealed.

The court papers detail for the first time how a decades-old cold case picked up momentum over the past year, with detectives focusing on a family that has roots in suburban Maryland and rural Virginia.

Neither the convicted sex offender, Lloyd Welch, 58, nor his uncle, Richard Welch, 69, has been charged in the case. In an earlier letter to The Washington Post, Lloyd Welch said he had “nothing to do” with the disappearance of Sheila Lyon, 12, and Katherine Lyon, 10. Richard Welch has declined to comment, but his daughter said that the allegations are a lie and that “my dad would never do something like that.”

For decades, the disappearance of the Lyon sisters has been one of the Washington region’s most painful unsolved crimes. To thousands of people, the case became a seminal event, the tragedy that convinced them it was no longer safe for children to walk alone — or that it never had been. Now detectives are increasingly hopeful that they can finally bring answers to the Lyon family, who for decades have lived a nightmare with no end.

In laying out the general contours of their case in the affidavits, detectives offered a narrative of heartbreaking violence – two girls abducted from a shopping mall, at least one of them sexually assaulted, both of them killed, their bodies disposed of on a remote mountain some 200 miles from their home.

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

Citing the ongoing investigation, Montgomery police officials declined to comment specifically on the affidavits or how credible they found Welch’s recent statements. Since October, an investigative grand jury has been meeting in Bedford County, Va., to take testimony from members of the Welch family — including Richard Welch — and others. Police also have been searching for the girls’ remains on the Virginia land and submitted the affidavits to support their search warrants.

“Our department and our partners in Virginia remain committed to determining what happened to Katherine and Sheila Lyon,” Montgomery Police Chief Tom Manger said Friday. “We believe that there are people, including family members of Dick and Lloyd Welch, who have information that would further this investigation.”

It was back on March 25, 1975, that Sheila and Katherine walked to Wheaton Plaza, looked at Easter decorations, ate pizza and browsed for clothes. When they didn’t come home, their parents called police. A massive search was launched — in woods, in ponds, even bringing in National Guard troops. But the girls had vanished.

Over four decades, detectives have pursued untold leads and periodically identified potential suspects. But they never charged anyone.

The affidavits do not specify whether any incriminating evidence has been found on the Virginia land being searched, in a section of Bedford known as Taylors Mountain. Investigators have found bones on the property, the search warrants state. But that may not mean much because the land contains an old cemetery with more than 30 unmarked graves. At one point during the search, detectives wrote, a dog trained to sniff decomposition “alerted” to areas away from the cemetery.

How Lloyd Welch surfaced in the case is detailed for the first time in the filings.

In May 2013, cold-case detectives came across a report of an interview investigators had with Lloyd Welch a short time after the sisters disappeared. It told an odd story.

Randy Krantz, commonwealth attorney for Bedford County, Va., asks other investigators about nearby landmarks while visiting a property in Thaxton, Va. Krantz and other investigators were looking at the property trying to determine if there may be a link to the dissapearance in 1975 of Sheila Lyon, 12, and Katherine Lyon, 10, who were last seen at a shopping mall in Wheaton, Md. (Norm Shafer/For The Washington Post)

On April 1, 1975 — seven days after the girls vanished — Lloyd Welch, 18 at the time, went to Wheaton Plaza and told a security guard that he’d seen the sisters get into a car with a man and leave. Detectives were called in and gave Welch a polygraph test. The results “deemed that Lloyd Welch was being untruthful in the interview,” according to the affidavits.

The affidavits did not say whether authorities continued to look into Welch, although police officials have said he had not been considered a suspect during the initial investigation.

The detectives on the case in 2013 set out to learn more about Welch. He had been arrested in 1977 in Montgomery in a burglary case. His mug shot from that arrest bore a “strong likeness” to a composite sketch in the Lyon sisters case, according to the affidavits. That sketch had been drawn in 1975 based on a witness’s description of a man at Wheaton Plaza who was staring at the girls and following them.

The detectives also found that Welch had worked as a carnival-ride operator, traveling across the country. In 1994, he had pleaded guilty to sexually assaulting a 10-year-old girl in South Carolina. In 1998, he pleaded guilty to sexually assaulting a 10-year-old girl in Delaware and was sentenced to three decades in prison. The detectives went to Delaware, according to the affidavits, and interviewed Welch several times in prison.

“During these interviews, Lloyd Welch has admitted he left Wheaton Plaza in a vehicle with the Lyon sisters on the day they disappeared,” according to the affidavits, which were written by Montgomery Detective Mark Janney and Detectives R.D. Baldwin and S.O. Smith of the Bedford County Sheriff’s Office.

In Lloyd Welch’s interviews with detectives, he said his uncle, Richard Welch, was involved in kidnapping the girls, according to the affidavits. He also said that another relative, a juvenile, was with them in the car that day. Lloyd Welch said he was dropped off near his home, and his two relatives continued on with the girls.

Lloyd Welch also said that the next day, he went to the home of Richard Welch and saw his uncle sexually abusing one of the sisters, according to the affidavit.

“Lloyd Welch claims that he left the residence and never saw the Lyons sisters again,” detectives wrote.

By last year, the suspicions of Montgomery detectives had grown so strong that police held a Feb. 11, 2014, news conference to name Lloyd Welch a “person of interest.”

Relatives of the couple in Bedford County later told detectives that after the news conference, Richard and his wife, Patricia, made several unannounced trips to the area. “Richard and Patricia Welch have also been extremely interested in whether these relatives were contacted by investigators,” detectives wrote, “and what information they provided to the investigators.”

On Dec. 5, Patricia Welch testified before the Bedford grand jury. What she said remains confidential. But afterward, she was charged with perjury.

Her attorney, Emmette Pilgreen of Roanoke, declined to comment on that charge. But he said his client wants to help: “She absolutely feels bad about the situation with these kids. It’s a horrible situation.”

Pilgreen suggested that Lloyd Welch is trying to divert investigators. “From what I understand, he is trying to point the finger on everyone but himself,” Pilgreen said.

That opinion is shared by another relative, Thomas Welch Jr., a family member who Lloyd Welch said was in the car during the kidnapping, according to the affidavits. In an interview Friday, Thomas Welch Jr. denied being in the car and said he was 10 1 /2 at the time. “I haven’t done anything,” he said.

Thomas Welch Jr., who lives in Calvert County, Md., said he doesn’t think that Richard Welch, his uncle, is involved, either. “I still don’t feel in my heart that my uncle had anything to do with this,” he said.

But he was hardly so sure about Lloyd Welch, his cousin, who he said had been considered a black sheep in the family. “If Lloyd did this, I hope he fries for it,” Thomas Welch Jr. said. A relative of the Welches told detectives that she recalled Lloyd Welch visiting Parker unannounced in the spring of 1975, according to the affidavits. When contacted by a reporter, that relative has declined to discuss the case.

In the affidavits, detectives described how they were able to link Lloyd and Richard Welch to adjacent parcels of land in the Taylors Mountain area of Bedford County. (Over the years, the area also has been spelled Taylor’s Mountain and Taylor Mountain.) One parcel is owned by Richard Welch. A larger parcel next to it was formerly owned by Allen and Elizabeth Parker, who is Richard Welch’s sister.

Anyone with information about the case can call Montgomery County police at 240-773-5070. A reward of up to $17,000 is available.