Authorities from Maryland and Virginia are set to announce what they call "significant developments" in the 40-year-old case of two sisters who vanished from a suburban Maryland mall.

Police and prosecutors from Montgomery County, Maryland, and Bedford County, Virginia, will hold a news conference Wednesday in Wheaton, the town from which the Lyon sisters disappeared in 1975. Katharine Lyon was 12, and Sheila Lyon was 10.

Police have named a sex offender, Lloyd Welch, and his uncle, Richard Welch, as persons of interest in the case. Neither man has been charged. Lloyd Welch has denied involvement in the disappearance. Richard Welch has declined to comment.

Authorities have been searching a mountain near Bedford, Virginia, for the sisters' remains and other clues since last year.