(CNN) -- Chris Watts' pregnant wife and two young daughters had disappeared that morning. Now he was in his neighbor's living room, pacing back and forth, watching himself on the screen. His neighbor's surveillance camera had captured Watts loading his truck earlier that same day -- with what Watts later admitted were the bodies of his wife, Shanann, and daughters, Bella, 4, and Celeste, 3. Now, the neighbor, Nathaniel Trinastich, was playing the footage for police while Watts watched.
The scene was captured in a police body camera video on August 13, part of a trove of newly released evidence in the case that provides a window into the moments leading up to and following the murders.
As Watts stands next to the television, he anxiously tries to explain away his every movement to those in the room. He was just loading up some tools for work, he says. A lunch box, too. Minutes later, after Watts left the house, Trinastich expressed doubts to police.
"I just don't understand why he keeps explaining himself over and over," Trinastich tells police. "He doesn't look worried. He looks like he's trying to cover his tracks."
The neighbor also provides police with a drastically different story about the seemingly picture-perfect couple next door.
"I've heard them full out screaming at each other at the top of their lungs, and he gets crazy," Trinastich tells police.
'It was like he had nothing to hide'
A newly released police interview with Watts' mistress also details how his behavior had changed in the days leading up to the murders.
During their investigation, authorities discovered Watts was having an affair with one of his co-workers. As Shanann Watts desperately tried to save her crumbling marriage, police say her husband was focused on this new love interest.
Investigative documents paint a picture of a secret relationship carried on with explicit text messages and frequent sexual encounters.
The mistress, identified as Nichol Kessinger, said she believed Watts was in the process of divorcing with his wife.
Speaking to an agent with the Colorado Bureau of Investigation, Kessinger says that Watts would often pay for dates with gift cards -- possibly so his wife couldn't track his spending. "He always paid with those. Always," Kessinger tells police.
But during their last date together the night of August 11, Kessinger says something changed. Instead of paying with his usual gift cards, Watts paid with a personal credit card.
"It was like he had nothing to hide. Or nothing to lose," says Kessinger.
Just days later, Watts' family would be found dead. Shanann was buried in a shallow grave, and the girls were found in large, 400-barrel oil tanks.
Weld County District Attorney Michael Rourke said he believes this was a crime Watts "absolutely thought" that he could get away with.
Watts pleaded guilty in November to first degree murder and other charges in their deaths and was sentenced to five life sentences with no possibility of parole.
(The-CNN-Wire™ & © 2018 Cable News Network, Inc., a Time Warner Company. All rights reserved.)
for more features.