A "drifter" by the name of Lloyd Lee Welch, Jr. was identified today by Maryland police as a person of interest in the 1975 disappearance of two sisters from a Montgomery County shopping mall.

Welch, also known as Michael or Mike Welch, was at Wheaton Plaza on the day Sheila Lyon, 12, and her 10-year-old sister Katherine vanished and was observed "paying close attention" to the girls, authorities said at a news conference.

Welch was employed by a traveling carnival company as a ride operator at the time, police said, and has multiple convictions for sexual offenses against young girls and has been incarcerated since 1997 in Delaware for one of these offenses.

Katherine, left, and Sheila Lyon vanished 39 years ago from a Maryland shopping center.

Police are hoping to find more information about Welch, who was considered a "drifter" and was in roughly "two dozen locations" from 1974 through 1997, including the Washington, D.C., area; North Miami Beach, Fla.; Los Angeles; Sioux City, Iowa; and multiple locations in South Carolina, police said.

News of the abduction rocked the community in the suburbs of Washington, D.C. It sparked several false leads, but the case went cold. Welch is the first significant person of interest in the 39-year-old case, police said.

"When we went back over all of the information that was accumulated at the time of the occurrence, looking at it with fresh eyes, having the ability to look at folks that we had established were at the scene and looking at where some of these folks are now, that was the lead that we were looking for," said Montgomery County Police Chief Tom Major.

Montgomery County Police mugshot of Lloyd Welch, 1977. He has been incarcerated since 1997. (Courtesy Montgomery County Department of Police)

Authorities are hoping that additional information on Welch's past may be able to connect Welch to the Lyon's disappearance. They urged anyone with tips to call (800) CALL-FBI or visit fbi.gov.

"We need everyone who may know something to come forward. Regardless of how insignificant it might seem, it might be enough to put the pieces of this puzzle together," said Steve Vogt, Special Agent in Charge of Maryland for the FBI.

"We know we're asking for people to remember things they may not want to think about. It's not easy to come forward if you've been assaulted or if you know someone who has," Vogt said. "You may have tried to put it behind you, but the information you provide can help lead us to information about what happened to Sheila and Katherine. They deserve justice, and the family deserves closure, just as you do."

Police are also looking for additional information on Welch's longtime girlfriend, Helen Craver, now deceased, as well as any security guards from the Westfield Plaza shopping center who may be able to provide additional details.

In a statement released today, the Lyon family said, "March 25 th will mark 39 years since Kate and Sheila were taken from our family. Throughout these years, our hopes for a resolution of this mystery have been sustained by support and efforts of countless members of law enforcement, the news media, and the community. The fact that so many people still care about this case means a great deal to us. We are grateful for any information the public can provide to help bring this story to its conclusion and ask that the family's privacy be respected during this time."

The girls' father, John Lyon, once a prominent radio personality at WMAL, is now a victims' advocate. Their brother, Jay Lyon, 15 years old at the time, became a detective with the Montgomery County Police.