Police in Maryland say a convicted sex offender currently imprisoned in Delaware has emerged as a person of interest in the 1975 disappearance of two young sisters who never returned home from a shopping mall.

Montgomery County Police Chief J. Thomas Manger says investigators have determined that the man was at the mall on the day the girls, Sheila and Katherine Lyon, went missing and was seen paying attention to them.

The man has criminal convictions in multiple states for sexually abusing young girls.

Manger identified the man as 57-year-old Lloyd Lee Welch. He said police are not ready to charge him but are seeking the public's help to gather more information.

An attorney for Welch could not immediately be found.

He declined to reveal exactly what led authorities to the man, but said he's only recently emerged as a person of interest.