The indictment of a suspect in the 40-year-old slaying of two sisters who were abducted from a suburban Maryland mall leaves prosecutors with a challenging task: proving that a man killed them when he says he didn't do it, no bodies have been found and other physical evidence has vanished.

Authorities announced Wednesday that Lloyd Lee Michael Welch Jr., a convicted child sex offender who's already serving a lengthy prison term, is facing murder charges in the deaths of 12-year-old Sheila Lyon and 10-year-old Katharine Lyon.

The sisters were last seen on March 25, 1975, at the Wheaton Plaza Mall near their home, where they went to shop for Easter decorations and eat lunch. According to the indictment unsealed Wednesday, they were kidnapped and taken to Bedford County, Virginia, about 200 miles away, where they were slain sometime between the date of their abduction and three weeks later.

Authorities began searching a rugged mountainside for the girls' remains last year but haven't found them. The search will continue, police said Wednesday, but prosecutors said they are prepared to bring Welch to trial even if they are never located.

"We have a difficult road. We have a difficult burden," said Randy Krantz, the Bedford County commonwealth's attorney. "But in my assessment of this case, it was either move forward or do nothing, and we were willing to move forward."

Prosecutors in the United States have obtained hundreds of convictions in murder cases without a body. Krantz and John McCarthy, the state's attorney for Montgomery County, Maryland, said they studied those cases as they weighed whether to proceed. The Lyon sisters' case presents an unusual burden because of the amount of time that has passed, but prosecutors said they are sure the girls were murdered.

"We know what Katharine and Sheila were like," McCarthy said. "The fact that they did not return home that day was because they were prevented from returning home. These were wonderful, wonderful, naive young children."

Tad DiBiase, a former federal prosecutor who tracks and has written a book about "no-body" murder cases, said that if Welch is tried, it would set a record for the longest period of time between a slaying without a body and a trial. Still, he said he believes a conviction is possible.

"They do have some damning evidence against him," DiBiase said. "You always want forensic evidence, but it doesn't look like they have that, at least so far."

Documents filed in court by cold-case detectives show that detectives missed an opportunity to further investigate Welch in the days following the slayings. At the time, police were focused on another potential suspect, an older man seen talking to the girls at the mall. Police released a sketch of that man to the news media.

According to a police affidavit, in 2013, a detective uncovered an interview that Welch had given to police in 1975, about a week after the slayings. The detective compared a 1977 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.

At the time of the slayings, Welch was an 18-year-old traveling carnival worker who had been spending time in the Wheaton area. A week later, according to the affidavit, he returned to the mall and told a security officer that he had been there the day the girls disappeared and saw them get into a car with a man. Police interviewed him and gave him a polygraph examination, which he failed, but they did not pursue him further as a suspect.

Detectives have interviewed him in prison several times since they began looking at him again as a possible suspect. According to the affidavit, he told police he left the mall with the girls in a vehicle on the day they disappeared. The next day, Welch told police he saw his uncle, Richard Welch, raping one of the girls at his home, according to the affidavit. Lloyd Welch said he left and never saw the girls again, according to the document.

Richard Welch remains a person of interest in the case, authorities said Wednesday. His wife, Patricia, was charged with perjury after testifying before a 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.

Lloyd Welch told police he didn't kill the girls, and he has also denied involvement in the slayings in a letter to The Washington Post. He has refused repeated requests by The Associated Press to interview him from prison in Delaware, where he is serving a decades-long sentence for molesting a 10-year-old girl.

He also pleaded guilty in 1994 to molesting another 10-year-old girl in South Carolina, and he served prison time after two burglary convictions.

At the news conference announcing the charges, police and prosecutors spoke of a "conspiracy" to conceal the slayings, but they did not detail how many people they believed to be involved. The investigation is continuing, and the grand jury that indicted Welch remains impaneled, authorities said.

Welch was charged with first-degree felony murder, committed during an abduction with intent to defile. Under the 1975 Virginia statute that he would be tried under, he could face the death penalty or 20 years to life in prison. Krantz said prosecutors have not decided whether to seek the death penalty.

The girls' parents, John and Mary Lyon, who still live in the area, attended Wednesday's 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.

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