Chris Watts, 33, is serving multiple life sentences after pleading guilty last November to murdering his family

Chris Watts, the Colorado man who murdered his pregnant wife and two young daughters, sits in a Wisconsin prison on 23-hour lockdown.

For one hour a day, he gets out of his cell for showers and exercise. For the past 40 days, his time out was staggered so that he didn’t interact with other prisoners. He remains in the evaluation unit of the prison.

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When he is in his cell, he has little to do: He is allowed to have a Bible, but not much more.

With so much free time on his hands, Watts has spent a lot of time thinking about his past, says a family friend who has asked to be identified only by her first name of Kim.

“He’s sad that everyone is hurting,” Kim tells PEOPLE. “He wishes he could go back in time. He wish he had handled things differently.”

Watts, 33, is serving multiple life sentences after pleading guilty last November to the August 13, 2018, murders of his pregnant wife, Shanann, 34, and the couple’s two young daughters: Bella, 4, and Celeste, 3.

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There is a chance that Watts could challenge his case under a very specific law known as Rule 35c.

In the Colorado Rules of Criminal Procedure, Rule 35c provides defendants with an opportunity to challenge their conviction and sentence. Because a 35c challenge is slightly different than an appeal, a defendant can raise questions about his conviction even if he has pleaded guilty.

Under Rule 35c, Watts must either prove that there is new evidence, or that his conviction was somehow unconstitutional.

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In a Feb. 18 interview from inside prison, Watts gave chilling details of the murders to authorities from the Colorado Bureau of Investigation.

He said his wife Shanann “may have been” praying as he strangled her to death inside their bedroom. He said he then drove his wife’s body to his job site, where he buried it in a shallow grave as his daughters waited in the car. Additionally, he claimed Bella saw him smother Celeste and screamed “Daddy, no!” at him before he killed her.

He also said he speaks to the photos of his family in his cell and reads a book out loud every night to the daughters he murdered.