The parents of Lauren Spierer, who have fought to keep their daughter's case in the public eye since she vanished a year and a half ago, appeared on Katie Couric's network talk show Monday, joining other families searching for relatives.

Robert and Charlene Spierer of Greenburgh appeared on a segment called "Vanished: Gone Without a Trace," highlighting the nation's 85,000 active missing-persons cases, more than a third of which involve children.

Lauren Spierer, a 20-year-old student at Indiana University at the time she vanished, was last seen June 3, 2011, leaving Kilroy's Sports Bar in Bloomington, Ind., after a night of partying with friends. Since then, the trail has gone cold for police and private detective Richard "Bo" Dietl, hired by the Spierer family.

On the Couric show, Robert Spierer criticized the friends who saw his daughter the night she disappeared.

"Here you have these boys who lawyered up, who clammed up, who shut down ... it's very frustrating," Robert Spierer said.

Lauren Spierer's parents have said publicly they believe that their daughter's friends know what happened the night she disappeared but won't talk to investigators because the facts could get them in legal trouble.

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Of particular concern, Robert Spierer said, was the refusal of his daughter's friends to take police-administered polygraph tests, though at least one of them took an independent polygraph.

"I'm frustrated and I'm angry at this point. We've been stonewalled by the last people who saw Lauren alive," he said.

"I don't think it was a random abduction," Charlene Spierer said. "I believe it was someone that Lauren knew."

The couple spent weeks in Bloomington after their daughter disappeared. They visited the area around Kilroy's during their time in the city and were appalled to find "young women walking alone ... as if what happened to Lauren didn't matter at all," Charlene Spierer said.

Hundreds of volunteers joined searches around Bloomington and in the surrounding countryside, which is dotted with old limestone quarries and lakes.

As to whether Lauren might still be alive, "The likelihood of her being alive is not very good," Richard Spierer said, but he added that he clings to "the dimmest of hopes."

"We really don't know what happened to her," he said.

The Spierers maintain an active Facebook page and Twitter account at #findlauren. The resiliency of their supporters is reflected on tweets from late November: "Day 545: Where is Lauren Spierer? Someone knows!" and "Please let today be the day to #FindLauren."

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Also on the show was John Walsh, host of the TV show "America's Most Wanted." His own son disappeared in Florida more than 30 years ago and eventually was found murdered.