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?"