Katherine, left, and Sheila Lyon. (Montgomery County police)

A sex offender accused in the 1975 disappearance and murders of sisters Katherine and Sheila Lyon from a Wheaton shopping mall — one of Maryland’s most notorious crimes — is poised to enter a guilty plea, according to online court records and officials familiar with the negotiations.

Lloyd Lee Welch, 60, charged two years ago, would become the first person convicted in the girls’ deaths, in a case that many feared would never be solved. Based on their recent work, detectives believe the sisters, ages 10 and 12, were sexually assaulted and killed before the remains of at least one of them were taken to rural Bedford County, Va., and burned.

Welch, a former carnival worker, had been set to go to trial beginning Tuesday in Bedford, but online court records show the trial has been “withdrawn.” A subsequent entry, now online, shows a plea hearing set for Tuesday.

Welch instead has agreed to a plea agreement that calls for him to admit guilt in the Lyon sisters’ disappearance and face decades in prison, according to multiple people with knowledge of the case. The terms, which have been under negotiation for several weeks, also would resolve unrelated sexual assault cases in Prince William County, according to people familiar with the agreement.

Any plea deal would not be final until Welch appeared before a judge and admitted guilt, and the judge signed off on it. He is set to appear in court in Bedford on Tuesday, and Welch could change his mind before the agreement is finalized.

Lloyd Welch in 1977 and 2015. (Maryland and Virginia law enforcement)

If Welch is convicted, it will bring a measure of justice for a family that has lived with the tragedy for 42 years. The sisters vanished during a March 25, 1975, outing to the Wheaton Plaza shopping center, where they had walked to see friends, look at Easter decorations and have lunch. Massive searches turned up no sign of Katherine and Sheila, and residents throughout the Washington region suddenly questioned the safety of their neighborhoods.

The crime went unsolved for decades before cold-case detectives homed in on Welch in 2013. He was indicted two years later.

The plea agreement would spare the Lyon family a long and difficult trial that could have included gruesome admissions about what happened to the girls.

And a plea means prosecutors would not have to piece together a case made difficult because the girls’ bodies were never found, key witnesses have died and there was little to no forensic evidence to bring forward.

Police and prosecutors continue to believe others were involved in the killings, but those potential suspects have died or officials felt they did not have strong-enough cases against them.

If the plea goes through, given Welch’s age and parole practices in Virginia, it is difficult to see him ever getting out of prison.

Welch is serving a long prison sentence in Delaware, where he pleaded guilty in 1998 to sexually molesting a 10-year-old girl. He is scheduled to be released in that case in April 2026.

Welch is not eligible for parole in Delaware. He could continue to earn good-behavior credits and quicken his release date to sometime around 2024, when he is 67. If he is convicted in Virginia under the plea, he would then be moved to a prison in the state to begin serving time for the Lyon sisters’ deaths.

Virginia abolished parole years go, but it remains in place for crimes committed before 1995 under a provision that grandfathered in those cases.

Under those rules, according to Virginia’s Parole Board Policy Manual, many convicts become eligible for parole consideration after a quarter of their sentence.

But someone with Welch’s criminal past who pleaded guilty to his current charges in Bedford would have a very difficult time getting released, said Adrianne Bennett, chair of the Virginia Parole Board.

After the initial investigation into the Lyon sisters’ disappearance slowed, it seemed the case might never be solved. Montgomery County investigators developed suspects over the years, but not with the certainty they needed to file charges.

The dynamic began to shift in 2013, when investigators took a closer look at Welch, whose name they noticed in an old case file. In 1975, when he was 18, he lived in Maryland and claimed to have seen the girls at the mall.

As detectives looked deeper, their suspicions grew.

Welch had picked up an extensive criminal record since the mid-1970s — in Maryland, Florida, Iowa, South Carolina and Delaware. The most serious cases were convictions for sexually assaulting a 10-year-old girl in South Carolina and another 10-year-old girl in Delaware. The detectives noted that he was serving time in Delaware and began to travel there. Welch spoke to them, and his words became a big part of the case against him.

In subsequent court documents and court hearings, police and prosecutors laid out in broad terms what they think happened to the Lyon sisters:

Welch began to follow the pair that March day, according to the accounts. Sometime after 2 p.m., he and others managed to get the sisters out of the mall and into a car — through deception, coercion or threats.

The girls were then held in Maryland for a brief period, sexually assaulted and murdered, according to the accounts of police. They believe Welch carried some of the remains to wooded land in Bedford County that had been owned by his family. He was spotted throwing at least one large bag onto a fire, according to court records.

In 2015, Bedford County prosecutors secured indictments against Welch for two counts of murder during “the commission of abduction with the intent to defile.”

Among law enforcement officials, the hope was always that Welch would give them specific and credible information that could lead to charges against others. But that never happened.

The plea agreement, according to people familiar with the case, also would resolve two other, unrelated cases against Welch for conduct in the 1990s — cases that arose recently during the Lyon investigation.

Last year, Welch was indicted in Prince William County, Va., accused of raping a 6-year-old girl on a houseboat in 1996. This year, also in Prince William County, he was indicted on charges of aggravated sexual battery, allegedly for abusing another girl around the same time, according to court records.