Drew Peterson's stepbrother said he believed the former cop was planning to murder someone in October 2007, but for the first time he described publicly his sickening realization that the person Peterson wanted to kill was his wife, Stacy.
Thomas Morphey recounts the moment he concluded that Peterson's target was his 23-year-old wife.
"All I could think ... was he was killing her while I was standing there," Morphey told "Good Morning America."
Morphey went to the police, won immunity for his involvement and disappeared for six months under police protection. But now, 18 months after Stacy Peterson vanished, Morphey said he had yet to be called by the grand jury investigating her disappearance.
"I just feel like the truth needs to be told at some point," he told "Good Morning America."
"I have to at least get it off my chest," Morphey said.
Morphey laid out for "GMA" two days of ominous conversations with Peterson that began Oct. 27, 2007 and convinced Morphey that his stepbrother was preparing to murder someone.
Stacy Peterson vanished Oct. 28, 2007, after reportedly telling her minister that Peterson had confessed to killing his third wife, Kathleen Savio, in 2004.
Morphey's chilling allegations against Peterson, 55, begin when the former undercover cop showed up at Morphey's home and asked him to get in the car for a ride to the park. On the way, Peterson asked Morphey, "How much do you love me?'" Morphey said.
"I said, 'I do.' And he said, 'Enough to kill for me?'" Morphey told "GMA."
"No, I couldn't live with myself," Morphey replied.
He said Peterson pressed on, asking, "Could you live with knowing about it?"
Morphey said he answered, "Yeah, I guess. We always figured you killed Kathleen," referring to Savio.
Peterson, looking disheveled and tense, told Morphey that Stacy was cheating on him, that he had caught her in restaurants with another guy. He had to do something about it, Morphey remembers Peterson saying to him.
From the park, Peterson drove to a storage facility where he wanted an increasingly nervous Morphey to rent a locker for him. He even offered Morphey $2,000 to put the locker in his own name, Morphey claimed.
"I said to him, Well isn't it going to smell? You know, what about the smell? And he said it would be in a sealed container," Morphey said. "I knew it wasn't good. He was planning on killing somebody."
Morphey said he assumed Peterson planned to murder Stacy's supposed boyfriend. "I didn't think for a minute he was going to try to kill her," Morphey claims.
Morphey said he was unable to rent the locker because he didn't have his ID with him.
A few hours later, Morphey tried to back out as Peterson's confidant in a possible crime, and he called Peterson and told him this was something he couldn't get involved in.
Peterson, he said, replied, "OK, I can respect that."
Despite Morphey's phone call, he said Peterson again showed up unannounced at Morphey's home the next day -- the day Stacy disappeared -- and took him for a ride to the park.
Once in the park, Morphey said, Peterson handed him a cell phone, told him not to answer it, then left.
Morphey said he paced back and forth, wondering, "Is he killing someone?"
The Mysterious Cell Phone Calls
About 45 minutes later the phone rang twice. Both times, the caller ID showed "Stacy's cell," Morphey said. The name on the phone shocked Morphey.
"Really, all I could think when I saw 'Stacy' on the phone was he was killing her while I was standing there," Morphey told "GMA."
Morphey said he was never certain whose cell phone he was holding.
"I believe it was Drew's phone. I believe Drew, more than likely had Stacey's phone. ... I believe he was setting up pings. Cell phone tower pings" to create a location for himself at the time of Stacy's disappearance, Morphey said.
According to Morphey, within an hour Peterson returned to pick up Morphey and to retrieve the phone. Morphey again tried to tell Peterson he had had enough.
"He said you got a minute? We need to run to the house. I need a hand with something," Morphey claimed Peterson said to him. "I wanted no part of going to his house, so I said no, I need to go home. Well, he said it would just take a second and he drove to his house."
Morphey told "GMA" that when they reached Peterson's house, the former Illinois cop went into his bedroom and emerged a few minutes later muscling a large blue barrel out of the room. He needed Morphey's help in getting the barrel down the stairs without tipping it.
He said the two men carried the barrel outside and put it in the back of Peterson's truck.
"I got in the car as soon as we left it there. ... I didn't even turn my head. I just looked straight ahead thinking ... my God, what's going on?" Morphey told "GMA."
When Peterson dropped Morphey off at his home, he said ominously, "This never happened," Morphey claimed. "I said I won't say a word."
When asked if Morphey knew where Peterson went with the barrel, he said, "I wish I did. Sometimes I wish I had rented that locker just because we'd know where she was."
But he also suspects Peterson's motives.
"On the other hand, I partially think that locker was to be put in my name so he could set me up," Morphey said.
Morphey was tormented by what he believed had taken place, possibly with his help.
"I knew I had had a part and granted, I didn't kill her. I helped him carry it and it just didn't sit good with me at all. I was beside myself," he told "GMA."
Morphey tried to commit suicide the next day by taking an overdose of pills. His wife drove him to the hospital, and a brother helped him go to the police.
Peterson has not been charged with any crime, but a grand jury has been convened to decide whether to indict Peterson for Stacey's disappearance. And Savio's death has been reclassified as a homicide.
Peterson maintains that his wife ran away with another man, and Morphey is concerned about what kind of case can be made, since the police don't have a body. So far, he has not been called to testify before the grand jury.
"I think they're doing everything in a timely fashion, and when they feel the time is right for me to go, I hope to be able to be there," Morphey said.
Peterson's Lawyer Scoffs at Morphey's Story
Peterson's lawyer Joel Brodsky has scoffed at Morphey's tale and cites his history of drug addiction and alcohol abuse.
"If they found him credible, [Morphey] would have been one of the first witnesses they would have brought in and they would have based the entire investigation and the entire case on his testimony," Brodsky said at a news conference called to rebut Morphey's first public allegations against his stepbrother.
Brodsky predicted that Morphey won't ever see the inside of a courtroom because prosecutors won't risk calling him as a witness.
Police investigating Stacy Peterson's disappearance found an e-mail to friends written before she vanished in which she confided to being in a "controlling, manipulative and somewhat abusive" relationship.
Peterson has said "that doesn't even sound like something she would say," adding, "I really question the origin of that e-mail."
Stacy's friend Pam Bosco claims to have been told by Stacy, "I love you. If anything happens to me, he killed me. It wasn't an accident."
ABC News' Martin Bashir, Eric Johnson and Katie Escherich contributed to this report.