Shelia Lyon, 12, and her sister Katherine, 10, went missing after a walk in 1975.
Montgomery County Police and the FBI released info on Welch on Tuesday.
Police in Maryland say a convicted child sex offender has been identified as a person of interest in the 1975 disappearance of two sisters who never returned home from a Montgomery County shopping mall.
Police have identified Lloyd Lee Welch, also known as Michael Welch, as the person of interest. Police say he was noticed paying attention to the sisters on the afternoon of their disappearance.
Welch has been in prison in Delaware since 1997, and has been charged with other sexual offenses against young girls. Police say he may have other victims who have yet to come forward.
"We have all been haunted by the disappearance of the Lyon sisters," said Montgomery County Police Chief Thomas Manger. "Even though so much time has passed, we have not forgotten that those young girls deserve justice, and their family deserves closure. We’re hoping one of you has information that will help us provide both."
Law enforcement officials are urging anyone with information on Welch to come forward or submit a tip to the FBI.
The announcement could represent a significant break in the investigation into the disappearance of 12-year-old Sheila Lyon and 10-year-old sister Katherine. They went missing after walking to a nearby shopping center.