Forty years after two young sisters vanished from a suburban Maryland mall, an imprisoned sex offender has been charged with murder, authorities said Wednesday, bringing some clarity to a baffling case that made parents afraid to let their children out of the house.

Lloyd Michael Welch Jr., 58, told authorities he was with the girls when they were abducted in March 1975 but denied any role in their deaths. Sheila Lyon, 12, and Katharine Lyon, 10, had walked a few blocks from their house to the Wheaton Plaza Mall to shop for Easter decorations and eat lunch. They never came home.

After decades of investigating leads and periodically identifying suspects, a break in the case came in 2013, according to an affidavit. A cold-case detective uncovered an interview that Welch gave to police in 1975. The detective compared a photo of Welch to a composite sketch from a witness who saw someone following the girls at the mall, and he saw "a strong likeness," the affidavit said.

Welch, who is serving a lengthy prison term in Delaware, was indicted by a grand jury in Bedford County, Virginia, on Friday on two charges of first-degree felony murder, though the girls' bodies were never found. The indictment, unsealed Wednesday, says the murders were committed during an abduction with intent to defile.

McCarthy and Randy Krantz, the commonwealth's attorney for Bedford County, said authorities believe the girls were slain in Virginia, although they did not detail the evidence that led to that conclusion. Authorities began searching a mountainside in Bedford County, about 200 miles from where the girls vanished, last year for their remains.

The girls were killed sometime between March 25, 1975 — the date of their abduction — and April 15, 1975, according to the indictment.

At the time of their disappearance, Welch was an 18-year-old carnival worker and drifter who had been spending time in the Wheaton area. Then, Welch told a mall security guard that he was at the mall the day of the girls' disappearance. Welch said he saw the sisters get into a car with a man. Welch spoke to police and failed a polygraph test, according to the affidavit, but investigators did not pursue him further as a suspect.

He was later convicted in two burglaries and served prison time. In 1994, he pleaded guilty to molesting a 10-year-old girl in South Carolina and was sentenced to 18 months. He is now serving a sentence for sexually abusing another 10-year-old girl.

Montgomery County Police mugshot of Lloyd Welch in 1977 Image: Montgomery County, Md., Police Department/Associated Press

In interviews from prison following the rediscovery of his 1975 interview and the sketch, Welch told police that he left the mall with the girls in a vehicle on the day they disappeared, and that the next day, he saw his uncle, Richard Welch, molesting one of the girls at his home, according to the affidavit. Lloyd Welch said he left and never saw the girls after that, according to the document.

Digging into Lloyd Welch, man indicted for murder of #LyonSisters. In 2003, he was part of inmate lawsuit suing over talking ban in chowhall — Chris Hurst (@chrishurstwdbj) July 15, 2015

Richard Welch was identified as a person of interest in the slayings last year but has not been charged. Richard Welch's wife, Patricia, was charged with perjury after testifying before the grand jury in December. A person who answered the phone at their home in Hyattsville, Maryland, on Wednesday said they weren't home and refused to take a message.

The girls' parents, John and Mary Lyon, who still live in the area, were at the news conference, but they left before it was over and did not speak to reporters. Police asked that their privacy be respected.

John and Mary Lyon, parents of missing sisters Katherine and Shelia Image: Molly Riley/Associated Press

If convicted, Welch could face 20 years to life in prison or could be put to death, Krantz said. He said prosecutors hadn't decided whether to seek the death penalty.

Today, the Washignton Post reported that Welch asked a relative in rural Virginia to wash bloody clothes that he was carrying in a duffel bag, according to another search warrant affidavit. Welch told the relative that the blood was from raw hamburger, but investigators believe that it could link him to the Lyon sisters' presumed deaths, The Post reported, citing the affidavit.

Indictment of Lloyd Welch leaves open possibility #lyonsisters were captive for 3 weeks before being killed pic.twitter.com/ZyP4CZOqi5 — Neal Augenstein (@AugensteinWTOP) July 15, 2015

Officials stressed Wednesday that the investigation remains active and more charges are possible. They credited a younger detective with connecting the dots in the case but declined to comment on any additional suspects or evidence and said they would make more information available Thursday.

Authorities spoke of a "conspiracy" to conceal the crimes but would not detail how many people they believe are involved.

"Noncooperation has prolonged this investigation and made it difficult," Krantz said.

Additional reporting by Mashable