In July, weeks before he strangled his wife and smothered their kids, Colorado dad Chris Watts sent love notes to the coworker he was secretly dating

Addictive, unique, wonderful — “an amazing, inspirational and electric woman.”

This is how Colorado father Chris Watts described his romantic infatuation this summer. But he wasn’t talking about his wife. He was gushing over his mistress, a coworker with whom he had begun an affair that, authorities believe, eventually drove him to murder his entire family on Aug. 13 in the pre-dawn darkness at their Frederick home.

Prosecutors have said Watts, 33, was thinking of a “fresh start” with a “new love” when he strangled his wife of nearly six years, 34-year-old Shanann Watts, with his bare hands and smothered their daughters Bella and Celeste, also likely with just his bare hands.

At the time of her murder, Shanann was 15 weeks pregnant with a boy to be named Niko.

The unbearably intimate murders and Chris’ calculating deceptions before his arrest — including pleading TV interviews about his missing family — attracted international attention.

Chris changed his story about what happened to his wife and kids several times, eventually claiming Shanann was responsible for their daughters’ deaths and that he killed her in a moment of rage.

But in November he admitted to all of his charges, in a plea agreement to avoid the death penalty, and will spend the rest of his life in prison.

“Why did this have to happen? His motive was simple, your honor,” Michael Rourke, the Weld County district attorney, said at Chris’ sentencing in November. “He had a desire for a fresh start and to begin a relationship with a new love that overpowered all decency and feelings for his wife and daughters and unborn son.”

Newly released documents in the case, obtained by PEOPLE, detail the extent of Chris’ passion for Nichol Kessinger, who worked with him at an oil and gas company.

Screen Shot 2018-11-26 at 12.04.27 PM_preview
From left: Nichol Kessinger and Chris Watts
| Credit: Weld County District Attorney's Office
Credit: Weld County District Attorney
Credit: Weld County District Attorney
Credit: Weld County District Attorney

In a July 3 card for her birthday, Chris wrote to Kessinger: “Big things will happen this year, Dreams will come true. That smile (that stare), that laugh (that giggle) gets me every time!! You are truly an amazing, inspirational and electric woman that takes my breath away every time I see you! You are wonderful! Don’t EVER stop being you!!!!”

Weeks later, in a July 30 note less than two weeks before he killed Shanann, Bella and Celeste, Chris wrote to Kessinger — “Nikki” — again. He made it clear he saw a future together with her.

“Wow, where do I even start?” he told her. “The first day I saw you, you took my breath away. The first day I had the guts to talk to you, I got lost in those stunning green eyes. The first day we hung out in the park together, I knew I was addicted.”

“The first time we kissed, I knew I had met the most amazing, unique and electric woman EVER!!” he continued. “We have a lot of FIRSTS together Nikki. And I want to keep having them with you!!”

Kessinger has said she was an unwitting mistress, and she came forward to police before Chris’ arrest. She first went public in an interview with the Denver Post after Chris pleaded guilty.

She told investigators they met at work either in May or early June and that Chris wasn’t wearing a wedding ring, according to an investigative report previously obtained by PEOPLE. While he told her he had children, Chris said he was separated from Shanann and living in the basement at their home, Kessinger said.

Kessinger said they began hanging out outside of work in late June or early July and talked often, the report states. She said she only learned Shanann was pregnant after Shanann and her girls disappeared — raising Kessinger’s suspicions.

“[She] thought if he could lie to her about this, what else would he lie about,” investigators wrote in the report.

Credit: Weld County District Attorney
Credit: Weld County District Attorney
Credit: Weld County District Attorney

Chris confirmed this basic account of the affair in his own interview with police on Aug. 15, not long before he was arrested. According to a report from his questioning, he said Kessinger “took his breath away and he never thought in a million years something like that could happen.”

Chris “said he never felt the same way about anyone in his lifetime as he did about ‘her,’ ” the report states.

For all of that infidelity, Chris’ leap from philanderer to killer remains somewhat unknowable to investigators and the Watts’ friends and family.

The two had been grappling with mounting marital stress, driven largely by Chris’ coldness as he embarked on his affair, but authorities have said he had no previous record of violence.

Kessinger, too, said she saw no red flags in his behavior.

Shanann Watts (center) and her daughters
| Credit: Shanann Watts /Facebook

“That’s the big word: why?” Shanann’s father, Frank Rzucek, told 20/20 this week. “He must have snapped, because there’s nothing else I could figure out what happened to him.”

Chris has not been required to make a public confession as part of his plea agreement, and prosecutors told PEOPLE they do not trust him not to lie again.

“I don’t expect that he will ever tell the truth about what truly happened or why,” Rourke, the district attorney, said in court in November.

Rourke said then that, even if Chris did come clean, “there’s no rational way that any human being could find those answers except those responsible to such horrific questions.”