At times patient, at times pushing, the cold-case detectives again went at Lloyd Welch inside the small interrogation room.

It was their 11th session with the longtime sex offender, who held answers to questions that had haunted a Maryland family for more than 40 years: What happened to Katherine and Sheila — the Lyon sisters — after they vanished from a shopping mall in 1975?

“I know I should be worried about the girls, the family, puttin’ it to rest and stuff like that,” Welch told the detectives. “But you also got to look at it, I’m a survivor. I’ve lived on the street. And like I told you, I’ve also gotta think of me. What’s going to happen to me?”

But soon, Welch was describing a gruesome story. In the days after the girls were abducted, Welch said, he’d gone into a dungeonlike basement, where he saw his father and an uncle dismember one of the girls. Her remains were put into a large bag, Welch said, which was taken to rural Bedford County, Va., and thrown into a fire.

His words, on May 12, 2015, further implicated Welch in the deaths of the Lyon sisters, to which he pleaded guilty Tuesday in Bedford, Va.

The conviction of the former carnival worker, after 42 years, marked an extraordinary moment in a case that stunned the region in 1975. The girls’ disappearance on a day when they had walked to a mall to have lunch, meet friends and look at Easter decorations at Wheaton Plaza became a seminal event for thousands of people, convincing them the world was no longer as safe as they had believed. That Kate and Sheila had seemed to vanish, and no culprits had been caught, enhanced the terror.

Welch’s plea to two counts of first-degree felony murder answered some but not all of the lingering questions in one of the Washington area’s most painful mysteries.

Left unknown is who, if anyone, besides Welch was involved in the Lyon sisters’ deaths, where they were killed and where the bodies are. Authorities have said other participants in the murders are either dead or their roles could not be proven.

“It keeps me up at night,” one of the investigators said recently.

Welch, 60, stood before a judge and admitted that he participated in the abduction of the sisters when he was 18.

He did not admit to directly killing either girl but was held accountable for their deaths under a felony murder doctrine for killings “in the commission of abduction with intent to defile.”

Welch received a sentence of 48 years in an agreement with prosecutors. Given his age, and that he still must finish a prison sentence in Delaware for the unrelated sexual assault of a 10-year-old, it is unlikely that he ever will be released. Welch was prosecuted in the Lyon sisters’ case in Bedford County — some 200 miles southwest of Washington — because authorities established that the remains of at least one may have been buried there.

Welch’s recollections of the murder in the basement that he relayed to detectives shed light on why so many questions in the case linger.

His father died in 1998. The uncle has denied any involvement and, after being investigated, was not charged. Detectives found what they thought was human blood in the basement and even a sample of DNA — but it wasn’t of the quality to make a match.

For the surviving Lyon family, haunted for 42 years about what became of their little girls, Welch’s admission may not have told them everything they wanted or brought the sisters’ remains home to rest, but it did mark an ending.

The family has remained intensely private about the case, although parents Mary and John, both 77, and their sons, Jay and Joe, were in court Tuesday.

In the rich voice he once used as a radio host — but halting at points from emotion — John Lyon thanked an array of Maryland and Virginia law enforcement personnel. Speaking specifically about cold-case detectives in Montgomery County, he said: “The last two or three years or so they have treated Sheila and Kate as if they were their own sisters or daughters. It’s been a long time. We’re tired and we just want to go home.”

In court, prosecutors outlined what they say happened, relying in part on Welch’s recollections of what he saw in 1975.

In the prosecutors’ narrative, the girls were abducted from the mall and killed. The remains of one or both were taken, by Welch, to land that his family owned in a rural part of Bedford County — and burned.

“How much you believe him [Lloyd Welch] really cut to the heart of this case,” said Wes Nance, the Bedford County commonwealth’s attorney. “His credibility is open for questioning. However, as the individuals that he named as his co-conspirators changed over time, what did not change was his involvement. . . . In my heart of hearts, I know that we put one of the main perpetrators away.”

As part of his deal, Welch also has agreed to plead guilty in two unrelated child sex assault cases in Prince William County, dating to the 1990s, that grew out of the Lyon sisters investigation, according to attorneys in the cases.

Welch has agreed to a 12-year sentence for those crimes, which will fold into the Bedford sentence and keep his total at 48 years. Under the plea, prosecutors in Montgomery County agreed not to pursue charges against him.

The day of the abduction "is the day we lost our innocence. We began to rear our children differently," said Montgomery County State's Attorney John McCarthy. "The entire region was affected by this case."

It also was the day that drew in "generations of cops who never stopped caring about this case," Montgomery County Police Chief J. Thomas Manger said in thanking them Tuesday.

Welch's lawyers, Aaron Houchens and Anthony Anderson, negotiated a plea agreement for their client that ended the uncertainty of possible prosecutions in Prince William and Montgomery counties.

Welch had limited intellectual abilities that made him easy to manipulate when he was younger, Anderson said, indicating that it may have played a role in Welch's decision to take part in abducting the girls.

"He didn't know they were going to be killed," Anderson said. "He knew that they were going to be exploited."

About five years ago, Montgomery County police decided to make one final push to solve the mystery. The approach: Let’s act as if a call had just come in for the two missing girls and scour the many boxes of case records as if starting from scratch.

One of the intriguing finds early on was a brief report — written by investigators a week after the disappearances — about an 18-year-old named Lloyd Lee Welch who had gone up to a security guard at the mall a week after the disappearance and said he’d been there on the day the girls went missing.

In that old account, Welch reportedly said he’d seen a man — referenced in a newspaper article around the same time — who was said to have talked to the sisters while holding a tape recorder.

Mall security called police, who administered a lie-detector test. Welch failed and was apparently dismissed by detectives at the time as an unreliable witness.

What the detectives who were newly plowing the case discovered was that Lloyd Welch later had compiled an extensive criminal record, including an arrest in 1977 in Montgomery County for a home burglary near the Wheaton mall and stealing $580 worth of jewelry. The burglary case yielded a mug shot, which bore a striking resemblance to a composite sketch drawn in 1975 of a man who witnesses said stared at the Lyon girls so intently at the mall that one of the girls' friends confronted him.

The newly assigned detectives learned that Welch was in a Delaware prison for sexually assaulting a 10-year-old girl in that state in 1997. Unsure of what to expect, they drove to see him.

Welch spoke to them — for eight hours.

He acknowledged that he was at the mall the day the Lyon sisters were reported missing, according to an affidavit detectives later submitted in court. When asked what happened to the girls, Welch said he believed they were "abducted, raped and burned up," according to the detectives' affidavit.

The investigators continued trying to uncover as much as they could about Welch.

They learned that his mother was killed in the crash of a car driven by his drunken father, Lee. Lloyd Welch was a passenger. He was placed in foster homes, ran away and started using drugs as a teenager, according to court records.

As an adult, he traveled the country and at one point started a landscaping business in South Carolina. In that state, he also was convicted of sexually assaulting another 10-year-old.

As the detectives gathered Welch’s history, they repeatedly returned to interview him. He would shift his story, offering names of relatives that he said he had seen abduct the Lyon girls. He also named relatives he said he had seen abuse and kill at least one of the girls.

In their visit on May 12, 2015, detectives tried to coax from Welch, in detail, that assertion of having witnessed a murder.

"This is me doing my job," Montgomery County Detective Dave Davis told Welch at the time, "and this is you trying to figure out a way to explain what you saw, what your involvement was and what we can prove and disprove. And I hope that makes sense to you."

"Yeah, it does," Welch said, according to court filings.

Welch started to describe a house where he said he had sometimes stayed in 1975 in the area of Hyattsville, Md., just outside of the District. His father and stepmother lived there, he said, and it had a concrete, dungeonlike basement with access only from an external door.

It was, like so many of Welch’s claims, gruesome in the extreme and yet teasingly credible.

Detectives and forensic technicians searched the basement in 2015, finding it just how Welch had described it. They drilled into concrete, trying to find patches of old blood but none was of high quality.

Welch’s father had already died. Detectives spent months listening to phone taps of the uncle who Welch said had been in the basement. They talked to people who knew him and probed his past before prosecutors determined that there wasn’t evidence to seek an indictment.

And that left only one person remaining in the account that Lloyd Welch had told:

Lloyd Welch.