The high-profile murder trial of Drew Peterson, a cop with the Bolingbrook (Illinois) Police Department, captivated America in September 2012. And it wasn’t only because he was a police officer accused of killing his third wife, Kathleen Savio—but because Peterson’s fourth wife, Stacy Peterson, had been missing since 2007.

Although Peterson was ultimately convicted of Savio’s murder and is currently serving 38 years in prison, he hasn’t been formally charged with Stacy Peterson’s murder, even though foul play is expected in her disappearance. (He’s also serving an additional 40 years in prison after being charged and convicted of solicitation of murder and solicitation of murder for hire after plotting to have Will County State’s Attorney Jim Glasgow killed.)

On an episode of ‘Marcia Clark Investigates the First 48,’ Clark investigates what may have happened to Stacy Peterson. She speaks to A&E True Crime about the evidence against Drew Peterson.

What do you think is the biggest piece of evidence in the case against Drew Peterson for the murder of his wife, Stacy Peterson?
I think they do have enough to convict, but I understand the concerns because a couple of key pieces of evidence hinge on the credibility of someone who has taken drugs in the past and was a little bit emotionally unstable at the time.

[Editor’s Note: While Marcia purposely doesn’t name the person she’s referring to in this interview, the person is, in fact, named on Marcia Clark Investigates The First 48. This person is a relative of Drew Peterson’s and, among other things, told police Drew had asked him, “Could you live with knowing that I killed someone?”]

But I absolutely believe [this person] told the truth [when he said] Drew Peterson, unbeknownst to him, had put Stacy Peterson’s body in a blue barrel and asked his help to dispose of the blue barrel.

Drew Peterson told this man that the blue barrel contained chlorine, but after the fact, he realized that it was probably Stacy’s body—key pieces of evidence [where] I think they need his credibility to really hold up. I think they were a little worried about that.

In my opinion, he was entirely truthful. I’m hoping eventually [the Will County State’s Attorney] will bring the case against Drew Peterson for the murder of Stacy Peterson.

Did your feelings on the case change while you reinvestigated it?
Yeah, they did. …It was a missing-persons case that I thought, the typical thing is given that this defendant killed his third wife and was convicted of it, the odds were that the fourth wife didn’t just disappear; she was killed too. But I wasn’t sure. I didn’t know how strong the evidence really was because that case never went to trial.

As I reinvestigated this and got into the facts of the case and all of the evidence, I realized it’s very strong and ultimately, extremely clear, in my opinion, that he killed her.

Related Features:

Marcia Clark: The Biggest Piece of Evidence Overlooked During the Casey Anthony Trial

Marcia Clark on Digging Up New Answers in Famously Complex Crimes

Watch the Drew Peterson Episode of ‘Marcia Clark Investigates the First 48’