Police on Tuesday identified a convicted sex offender as a person of interest in a nearly 40-year-old cold case involving the disappearance of two sisters in Montgomery County.

Lloyd Lee Welch, 57, a ride operator for a carnival company who often set up at malls, was noticed paying attention to the sisters the afternoon they disappeared near a shopping center in Wheaton, police said.

Sheila and Katherine Lyon have not been seen since March 25, 1975, when they were 12 and 10. They had shared a pizza at the Orange Bowl in the Westfield Wheaton mall, formerly known as Wheaton Plaza, about a half-mile from their home before they went missing.

Their disappearance has been among the most high-profile unsolved cases in the region, drawing a massive manhunt decades ago and a number of false leads in the years since the children were last heard from.

The Lyon family issued a statement Tuesday thanking law enforcement and the media for their continued interest in the case and asking that their privacy be respected.

“The fact that so many people still care about this case means a great deal to us,” the family said in the statement.

Police at a news conference Tuesday provided few details on how they developed Welch as a person of interest and how they placed him at the mall on the day of the disappearance.

Montgomery County Police Chief J. Thomas Manger said detectives developed the lead “recently,” when they looked back at lists of people they had established were at the mall on the day the girls went missing and where some of those people are now. He said detectives focused on Welch after reviewing his criminal history.

Welch has been in prison in Delaware since 1997, and Chief Manger said he had also been arrested and charged with sexual offenses in Virginia and South Carolina. Welch was arrested for a residential burglary in 1977 in Montgomery County, just a few blocks from Wheaton Plaza and served a prison sentence in Maryland from 1982 to 1984.

Police said Welch was originally from the D.C. area, but traveled extensively throughout the United States from the 1970s through the mid-1990s. They characterized him as a drifter who often stayed at homeless shelters.

Chief Manger noted the resemblance between a 1977 mug shot of Welch and a sketch artist’s drawing of a person described by a witness as being at the mall and paying attention to the girls on the day they vanished.

The chief declined to talk about any contacts police have had with Welch since he emerged as a person of interest in the case. He put out a wide-ranging plea for the public’s help in providing information about Welch or a girlfriend with whom he used to travel. The woman, Helen Craver, worked with Welch for the carnival company and is now deceased. Police also asked for information about former security guards and personnel at the mall who might have some knowledge of Welch.

Asked whether police currently have enough evidence to charge Welch, Chief Manger said, “If we were able to charge someone, we would have done it.”

The chief referred to a list of two dozen locations at which they have placed Welch from 1974 to 1997 and said additional victims might not yet have come forward.

“I think one of the things that we’re hoping for is that, based on his criminal past, his convictions, we believe that there may be other cases in these other states — other locations where he was during that time — that we may be able to connect him to. And that’s going to be helpful to us in terms of us dealing with this case,” he said.

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