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Kaelin, now 56, sat down for an interview with Barbara Walters for her Investigation Discovery series, “Barbara Walters Presents American Scandals," which airs Monday at 10 p.m. ET. He opened up about testifying at Simpson’s trial and how his feelings have changed about Simpson over the years.
“In my opinion, yes, I think he’s guilty,” Kaelin told Walters in a preview of the interview given to “Nightline.” “In hindsight of everything, like 20 years later, I think that O.J. Simpson is guilty.”
Watch more on "Nightline" tonight at 12:35 a.m. ET
At the criminal trial, Kaelin served as a witness for the prosecution and his testimony made him an overnight celebrity.
“If I saw O.J. today, I would say, ‘O.J., did you do it?” Kaelin said. “I think he would say, ‘absolutely not.’”
Kaelin, a friend to both O.J. Simpson and Nicole Simpson, helped establish a timeline for investigators. He said at the time that he had shared a takeout meal with O.J. Simpson on the night of the murders.
“O.J. Simpson came by my door in my guest house and he knocked on my door … and we ended up going to McDonald's,” Kaelin said.
Based on Kaelin’s testimony, prosecutors argued at the trial that O.J. Simpson and Kaelin returned from getting food at 9:36 p.m. on the night of the murders. Kaelin went to his bungalow and Simpson went to his house, and then an hour and 10 minutes later, a limousine arrived to take Simpson to the airport.
But in talking with Walters, Kaelin said he was with Simpson when the limo arrived.
“I was helping pack stuff into the limo because he was going to Chicago,” Kaelin told Walters. “And there was a bag that was ready to be packed, and he said ‘Don’t touch that bag,’ and that bag was never found."
“It looked like a backpack,” he continued. “I don’t know what was in there, but something in there was enough for O.J. Simpson to say, ‘Don’t touch.’”
When the verdict came back as “not guilty,” Kaelin said he remembered thinking the jury made a mistake.
Although Simpson was found not guilty in the much-disputed 1995 criminal case, he lost a wrongful death civil suit brought against him by the families of Nicole Brown Simpson and Ron Goldman in 1997. He was ordered to pay $33.5 million in damages, of which Goldman's father Fred Goldman told ABC News “20/20” this year that Simpson has not paid a single penny.
In September 2007, Simpson was arrested in Las Vegas for armed robbery after attempting to steal sports memorabilia he claimed belonged to him. Thirteen years to the day after being acquitted in his criminal trial, Simpson was found guilty of robbery and kidnapping and given a sentence of up to 33 years in prison.
Simpson, now 68, is serving time at Lovelock Correctional Facility in Lovelock, Nevada. He will be eligible for parole in October 2017.
“O.J. Simpson, he’s very much a character out of a Shakespeare play,” Kaelin told Walters. “Here’s a man who had everything and lost everything and became from adored athlete to pariah.”