WHEATON, Md. (AP) — Authorities on Wednesday announced first-degree murder charges against an imprisoned sex offender in the disappearance of two sisters from a suburban Maryland mall in 1975, bringing some clarity to the baffling case that made parents question whether to allow children out of their homes alone.

Twelve-year-old Sheila Lyon and 10-year-old Katharine Lyon walked from their house to the Wheaton Plaza Mall in March 1975. They never came home. No bodies were found.

After decades of investigating leads and periodically identifying suspects, officials say they now know who was responsible: Lloyd Lee Michael Welch Jr., 58, a child sex offender serving a lengthy prison term in Delaware. He had previously been named a person of interest in the case.

Welch was indicted on the two charges of first-degree felony murder with intent to defile by a grand jury on Friday, officials said. The indictment initially was sealed.

"We know what Katherine and Sheila were like. ... These were wonderful, wonderful, naive, young children," said John McCarthy, state's attorney for Montgomery County, Maryland.

At the time of the girls' disappearance, Welch was an 18-year-old carnival worker and drifter who had been spending time in the Wheaton area. Authorities have established that he was at the mall the day the girls vanished and was seen paying attention to them.

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

"This was something that had an enormous impact on this community and the feeling of safety for your children in this community," Montgomery County police chief Thomas Manger said.

Since last year, authorities have been searching a mountain in Bedford County, Virginia, about 200 miles from Wheaton, for the sisters' remains. A grand jury in that county handed down the indictment, and county Sheriff Mike Brown made announced the charges Wednesday.

Officials believe the girls were abducted in Maryland and killed in Virginia, prosecutors in both states said.

If convicted, Welch could face 20 years to life in prison or could be put to death, said Randy Krantz, commonwealth's attorney in Bedford County. He declined to elaborate.

Lloyd Welch had been named a person of interest along with his uncle, 70-year-old Richard Welch. Richard Welch's wife, Patricia, was charged with perjury after testifying before the grand jury in December.

Lloyd Welch denied involvement in the girls' disappearance in a letter to The Washington Post. Richard Welch, who lives in Maryland, has declined to comment.

According to police affidavits, Lloyd Welch told investigators that he left the mall with the two girls and that he saw his uncle sexually assaulting one of them at his home the next day. Lloyd Welch told investigators he left the home and never saw the girls again, according to the affidavits.

The Post reported that Welch asked a relative in rural Virginia to wash bloody clothes that he was carrying in a duffel bag, according to a 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.

Officials stressed Wednesday that the investigation remains active and more charges are possible. They declined to comment on any additional suspects or evidence and said they would make additional 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.