Lloyd Welch, the prime suspect in the 1975 case of two missing Maryland sisters, has revealed to authorities that the Lyon sisters were kidnapped so they could be sexually abused, investigators said at at a news conference Thursday.

"The motive of the abduction of the Lyon sisters was to sexually exploit, abuse and defile them," Bedford County, Va., Sheriff Mike Brown said, according to CNN.

Katherine, 10, and Sheila Lyon, 12, disappeared in March 1975 after they went to the Wheaton Plaza Shopping Center in Maryland to see an Easter exhibit and grab a slice of pizza. Investigators had not made any progress in the case until 2013, when police detectives decided to reopen the investigation.

Welch told investigators that he saw his uncle, Richard Welch, sexually assault one of the girls at Richard Welch's residence shortly after they abducted the sisters, Brown said, according to the Washington Post.

Investigators told reporters that they are not solely relying on information from Lloyd Welch in their investigation of Richard Welch, 70, who has not been charged.

Lloyd Welch, 58, was previously named as a person of interest in the case but was never charged. He is a child sex offender serving a prison term in Delaware, and he has now been indicted by a Virginia grand jury on two counts of first-degree felony murder. The process to extradite Lloyd Welch to Virgina began Thursday morning, according to CNN affiliate WDBJ7.

Search warrants in the case were unsealed on Wednesday, revealing that relatives told investigators that Lloyd Welch suddenly showed up at the family's Taylor's Mountain property in 1975 with a duffel bag of bloody clothing that he later burned.

Brown believes that the indictment is a "significant milestone in solving the case" and that officials will continue to work toward prosecuting anyone involved in the kidnapping and killing of the Lyon sisters.

The investigation was dubbed as "Operation Worthy Cause" in Montgomery County, Md. Bedford County Sheriff Mike Brown said a new phase of the investigation has begun, called "Operation Determine Justice."

Investigators still hope that witnesses will come forward, encouraging them to call 434-534-9521.