Convicted serial killer Scott Kimball's sons break silence 20 years after father's killing spree

Nearly two decades after authorities began connecting the dots to uncover the grisly truth behind four missing people linked to Scott Kimball, a Colorado FBI informant and serial killer, his sons are breaking their silence.

Kimball’s sons, Justin and Cody Kimball, who were just children at the time of the killing spree, are discussing for the first time details around what they claim is their father’s attempt to kill Justin when he was just 10 years old.

“I just remember thinking, ‘This guy is going to kill me, and he's obviously making it look like an accident. No one's going to know because I'm going to be dead,’” Justin, now age 29, said in an exclusive interview with “20/20's" John Quiñones.

One evening in July 2004, Scott Kimball and his two boys were out in the backyard digging holes and chasing mice. Justin said that when Cody went inside the house, his father told Justin to dig a hole in a specific spot while keeping his eyes on the horizon.

"[It] felt like I saw a bunch of stars, flashes in front of my face, and then came the big wham," Justin said. "I heard it hitting me."

A large metal cattle grate had fallen onto his head. While in route to the hospital, Justin claims his father tried to kill him again by pushing him out of a moving car.

“What I do remember, him pushing me out was by my face, because I remember how big his hand was and how warm it felt on my cold face,” Justin told “20/20.”

According to a police report, Scott Kimball told officers he was inside when Justin was playing near the grate, looked out and saw it had fallen on top of his son. Kimball also told police that he was trying to pull Justin back into the car, not push him out on the way to the hospital.

“At this time we have no reason to believe there was any criminal activity involved,” the police report stated.

The boys’ mom, Larissa, then divorced from Scott Kimball, rushed to the hospital when she got the news about her son.

“The surgeon came in, and he said, ‘I just want to tell you right now, it doesn’t look good,’” Larissa, who is using only her first name for privacy reasons, told “20/20.”

Justin suffered critical brain damage and was put into a medically induced coma, but miraculously survived. And his first words upon regaining consciousness would shock the family members gathered around his bed.

“I remember saying, ‘My dad did it.’ Because I remember thinking that my last thought before I lost consciousness was, ‘No one’s ever going to know that he tried to do this to me,’” Justin said.

According to authorities, there was a life insurance policy on Justin worth $50,000. Just days before the incident, Scott made himself the sole beneficiary.

When asked about how she felt when she learned about Scott's change to the policy, Larissa said, "To be honest, I think I threw up, because then it all started making sense. This was no accident."

At the time, the neurosurgeon treating Justin said the injuries likely affected his memory of the incident. Scott Kimball was never charged with any crime surrounding the incident with Justin, and he denies causing his son any harm.

Two years later, in 2006, local detectives in Lafayette, Colorado, began investigating Scott Kimball for check fraud. He was apprehended after a dramatic police chase in California and charged as a habitual offender, with previous non-violent crimes including forgery, theft and fraud.

However, the detectives suspected Scott Kimball was involved in much more than financial crimes, after it came to light that he was the last person to be seen with two missing women — Kaysi McLeod and Jennifer Marcum, who both disappeared in 2003.

In 2003, Kimball convinced the FBI to use him as a paid informant to stop a murder again. The FBI had used Kimball in the past as an informant in Alaska and Seattle, where they believed he was instrumental in helping to stop the murder of a federal judge and a prosecutor.

But in 2003, he not only falsely led the FBI to believe he could help them prevent murder as an informant in Colorado, he killed several people during this time period, including Jennifer Marcum. The FBI investigated her disappearance, but her body was never found and the case went cold. The FBI would later admit to being duped by Kimball.

Not only did Kimball deceive the FBI, but he also deceived Lori McLeod, the mother of one of the missing girls, Kaysi McLeod. Kimball explained that he worked for the FBI and was using his contacts to look for Kaysi.

Kimball's web of lies and manipulation finally unraveled in 2006, when the two fathers of those missing women went to the FBI. Rob McLeod had reached out to Bob Marcum after seeing a news article about Jennifer Marcum’s disappearance that mentioned Scott Kimball as being the last person to see her alive.

"Rob McLeod and Bob Marcum...come into the FBI office in November 2006," former Special Agent Johnny Grusing told ABC News. "Both Rob and Bob tell my boss that Scott Kimball took their daughters."

Scott Kimball also was connected to two more missing people, his own uncle, Terry Kimball, and LeAnn Emry, the ex-girlfriend of another one of Kimball’s former cellmates.

“Nobody saw a big picture of who Scott Kimball was. It wasn't until we started looking at his criminal history and piecing things together that we could see that each individual agency had no idea what they were really dealing with,” said Gary Thatcher, chief investigator with the Boulder County District Attorney’s Office, who was investigating Kimball for check fraud.

Investigators got a break in the case while questioning Kimball about the disappearances, and he mentioned the possibility that one of the missing women might be found on National Park Service land.

“Scott was trying to get time in federal prison because it’s easier life for him than in state prison,” Grusing said.

Investigators already knew that Scott Kimball said he had been hunting on the day Kaysi McLeod disappeared. While executing a search warrant of Kimball’s belongings, Grusing had found a receipt for a grocery store in Walden, Colorado, which is surrounded by National Forest land.

Grusing called the Forest Service and learned that a hunter had recently found a skull on the ground in the area.

“Based upon what Scott had said, based upon the receipt, based upon the godforsaken place this hiker was recovered, I had a really good notion it was Kaysi,” Grusing said.

Grusing was right, and the confirmation was a turning point in the investigation.

Now that investigators had recovered Kaysi McLeod’s remains, they negotiated a deal with Scott Kimball. He agreed to lead them to Jennifer Marcum, Terry Kimball and LeAnn Emry in exchange for reduced charges. Kimball’s information led investigators to the bodies of LeAnn Emry in Utah’s Book Cliffs, and Uncle Terry near Vail, Colorado. However, Jennifer Marcum was never found. As a result, his plea deal was renegotiated.

In October 2009, Scott Kimball was sentenced to 70 years in prison after pleading guilty to the murders of Kaysi, Jennifer, LeAnn and Terry. At the sentencing, the courtroom was filled with victims’ relatives.

“My daughter was a young woman with feelings and dreams and to treat her like trash is despicable,” said LeAnn Emry’s father, Howard.

Justin Kimball is affected by the grave injuries he says were inflicted by his own father and remains haunted by the killing spree.

“He’s unforgivable, and he’s exactly where he belongs,” Justin said.

"20/20: Rocky Mountain Horror" airs Friday, May 12 at 9 p.m. ET on ABC.

ABC News' Tami Sheheri, Lindsey Schwartz, Tim Gorin and Kyla Milberger contributed to this report.