Police identified a person of interest in connection with the 1975 disappearance of two young girls from a Maryland shopping center, a mystery that has haunted the D.C. area for almost four decades.

The Lyon sisters -- Sheila, 12, and Katherine, 10 -- were on their way home from the Wheaton Plaza Shopping Center in March 1975 when they disappeared. A composite sketch was widely distributed and tips flowed in, but the Lyon sisters were never seen or heard from again.

Now, a cold case squad has identified 57-year-old Lloyd Lee Welch Jr., aka Michael Welch, as a person of interest in the case.

Welch, a convicted sex offender in prison in Delaware since 1997 on a rape conviction, was noticed paying attention to the sisters the afternoon they vanished, investigators said.

Montgomery County police hope the public can help finally solve the case. At the moment they do not have enough evidence to charge Welch in the disappearance of the girls.

"Even though so much time has passed, we have not forgotten that those young girls deserve justice, and their family deserves closure," Montgomery County Police Chief Thomas Manger said. "We’re hoping one of you has information that will help us provide both."

Police hope the release of Welch's picture will jog someone's memory and help them find out what happened to the girls.

Originally from the D.C. area, Welch traveled extensively through the U.S. from the 1970s through the mid-1990s with a carnival company he worked for with his girlfriend Helen Craver, police said.

Welch's stepmother, Edna Welch, told News4 she recalls Welch and Craver hearing about a reward in the case and Welch saying he was going to try to collect it.

"Lloyd and Helen were sitting on my sofa when the news broke about the girls, and they were talking about a reward for information," Edna Welch said. "Lloyd thought he was going to get something in it, I reckon, and he called in and said he knew something, but I don't think he knew a darn thing."

Welch was charged with raping juveniles in Virginia and South Carolina. He was also arrested in a burglary not far from Wheaton Plaza. He was known to hitchhike throughout the D.C. area.

The Lyon family released a statement thanking people for their interest and concern and asked that the media respect their privacy at this time.

Shaking Our Sense of Safety

The disappearance of the Lyon sisters dominated local Washington news when it happened, and in the years since the mystery has become among the region's most famous cold cases. It inspired Baltimore mystery novelist Laura Lippman's 2007 book, "What the Dead Know," and was the subject of countless unsolved-crimes shows.

But what many people who grew up in suburban D.C. remember is how profoundly the disappearance shook their view of the safety of their community.

"It was just stunning. It could have been anybody's kids," said Charleen Merkel, a shopper at Westfield Wheaton who said she remembers the disappearance well.

"It brings back a lot of memories of being scared growing up," said another shopper, who did not give her name.

In an era when children frequently walked to school and elsewhere alone, parents started keeping their children inside.

The girls left their Kensington, Md., home March 25, 1975, to walk the half-mile or so to Wheaton Plaza, now known as Westfield Wheaton mall. They were on spring break, and wanted to get some pizza for lunch and see the Easter decorations. They had less than $4 with them.

A friend of the girls saw them outside the Orange Bowl restaurant with an older man who had a tape recorder and a briefcase, according to news and missing persons reports. A sketch of that man was made and distributed, but the man was never located.

The girls were later spotted on the road to home, but did not arrive by their 4 p.m. curfew. By 7 p.m. that night, police had been called.

A lengthy search continued but never turned up any concrete leads.

Retired Sgt. Harry Geehreng, one of the first detectives at Wheaton Plaza after the call went out, said police were swamped with calls and everyone in the bureau worked the case, but the leads all led nowhere.

At one point, the Center for Missing and Exploited Children produced age-progressed pictures of the girls; today, Sheila would be 51; Katherine would be 49.

A 'Hit-Home Case'

In interviews for the 30th anniversary of the disappearance, police spoke about the frustration of never being able to solve a case that struck such an emotional chord for the community and for themselves.

The girls' older brother, Jay, became a police officer.

"It's a hit-home case," Philip C. Raum, a longtime law enforcement officer in Montgomery County who headed the police's Major Crimes Unit for four years, told Montgomery County's Gazette newspaper in an article on the 30th anniversary of the disappearance.

Mystery novelist Lippman commented on the Lyon case in a online chat with readers on Goodreads.com. "The story ... happened when I was a teenager, not much older than the girls who disappeared (the Lyon sisters) and living in a similarly 'safe' suburb," Lippman wrote. "It resonated very deeply with me."

The girls' father was John Lyon, a popular radio personality on WMAL in Bethesda and performer.

"It's in that group of moments where the community just held its breath," radio personality Chris Core told the Gazette in 2005. He had just started working at WMAL with John Lyon when the girls disappeared.

"Partly because John was a well-known celebrity and partly because here are two innocent little girls going to the mall and disappear off the face of the earth, never to be heard from again."

Stay with News4 and NBCWashington for developments in this case.