The ex-girlfriend of a Colorado man who pleaded guilty to murdering his pregnant wife and their two young daughters described how he lied to her throughout their relationship and sent a chilling text message the day his family disappeared.

Nichol Kessinger, 30, briefly dated Christopher Watts over the summer after meeting at their job, Anadarko Petroleum. She ended the affair in August after his wife, Shanann, and his two daughters, Bella, 4, and Celeste, 3, went missing. Shanann was 15 weeks pregnant at the time she vanished.

Kessinger said she never doubted he killed them and even went to investigators with her suspicions.

In her first interview, she told The Denver Post that her short romance with Watts — which lasted less than two months — was plagued with lies, starting with him falsely telling her that he was close to finalizing his divorce from Shanann and planned to find a new apartment for him and his daughters.

“He made me believe that he was doing all of the things that a rational man and good father would do,” she said.

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Those lies started to unravel on Aug. 13, the day Shanann and her daughters vanished from their home. Watts, who publicly pleaded for his family to come back, told Kessinger that Shanann had taken the girls to a play date and never returned, she told The Denver Post.

She said Watts had also texted her the day his family vanished saying they were "gone."

Kessinger recalled how Watts showed little emotion while his family was missing and appeared calm. She said it wasn't until later that day that she learned troubling details about the disappearance, like Shanann's wallet still being at the home and how concerned people looking for the mother and children could not reach them.

Kessinger said it was only when she read the news that she found out his wife was pregnant.

“I thought, ‘If he was able to lie to me and hide something that big, what else was he lying about?’" she told the paper.

Kessinger began questioning Watts about his family's disappearance, but he would try and change the subject, telling her that he would never hurt his wife and kids.

“It got to a point that he was telling me so many lies that I eventually told him that I did not want to speak to him again until his family was found,” she said.

As her suspicions grew, Kessinger reached out to the Weld County Sheriff’s Office to tell them about their relationship and the lies Watts had told her.

Watts, 33, was arrested on Aug. 15 for the murders. He pleaded guilty on Nov. 6 to all nine counts against him, including five counts of first-degree murder, three counts of tampering with a deceased human body, and one count of unlawful termination of a pregnancy. He took the plea deal to avoid the death penalty. A sentencing hearing for Watts is scheduled for Monday.

“I don’t think there is a logical explanation for what he did,” Kessinger said. “It’s a senseless act, and it’s horrific.”