A 60-year old convicted sex offender has been sentenced to two 48-year prison terms in the killing of two young sisters who disappeared from a Maryland shopping mall more than 40 years ago.

RELATED: Man accused of Lyon Sister murder expected to plead guilty

Lloyd Lee Welch Jr. pleaded guilty Tuesday to two counts of first-degree felony murder in the 1975 killings of 12-year-old Shelia and 10-year-old Katherine Lyons. He was sentenced during the same hearing. Welch also received a 12-year sentence in two unrelated sexual assault cases in Prince William County in Virginia.

The Lyons sisters vanished on March 25, 1975, after walking to Wheaton Plaza, near their home in Kensington, Maryland.

A massive search yielded few clues. It took authorities nearly four decades to focus on Welch. By then, he had accumulated a long criminal history and was serving a prison sentence in Delaware for sexually molesting a 10-year-old girl.

"This has been a very, very long journey," officials said during a press conference Tuesday morning. Authorities added that this hasn't been an easy time for the family members of the Lyon sisters.

The chief manager released the following statement:

“The prayers of countless Montgomery County police officers, past and present, are with the Lyon family today. Our community still grieves their loss. With this plea, we can all be assured that Lloyd Welch will never be free to victimize another child or destroy another family.

I am eternally grateful to the Prosecutors both here and in Virginia. I am grateful for the generations of cops who never stopped caring about this case, and I am most grateful for the Lyon family for their strength that inspired us all.” – Chief Manger