Parents of Girls Slain in 1975 Thank Police

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Lyon sisters Lyon sisters

The parents of two young Maryland sisters who disappeared in 1975 have expressed their gratitude to investigators after a convicted sex offender pleaded guilty in their long-unsolved killings.

John and Mary Lyon thanked authorities in Maryland and Virginia Tuesday after Lloyd Lee Welch Jr. pleaded guilty to two counts of first-degree felony murder in the deaths of 10-year-old Katherine and 12-year-old Sheila Lyon. The two girls vanished after walking from their home to a local shopping mall on March 25, 1975.

The girls' disappearance haunted people who lived in the suburbs of Washington, D.C., for decades.

Welch was charged in 2015 after cold case detectives noticed a striking similarity between a sketch in the case file and a mugshot of Welch from the late 1970s.

He was sentenced to 48 years in prison Tuesday as part of a plea agreement with prosecutors that also calls for him to plead guilty to two unrelated sexual assaults in northern Virginia. Under the plea agreement, he will receive a 12-year concurrent sentence for those crimes.