"We believe that we have a partial motive," prosecutor Michael Rourke said after Chris pleaded guilty to murdering his wife, Shanann, and two children

Why Did Chris Watts Murder His Wife & Kids? D.A. Has 'Partial Motive' — but Can't Release It Yet

With Colorado father Chris Watts‘ guilty plea on Tuesday, it’s no longer legally in dispute who killed his wife of nearly six years, Shanann Watts, and their young daughters, Bella and Celeste.

But another question remains: Why did he do it?

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“We believe that we have a partial motive,” Weld County District Attorney Michael Rourke told reporters later Tuesday, after Chris, 33, pleaded guilty to nine charges including first-degree murder.

“I’m not sure if it’s the entirety of the motive, but I will be happy to comment on that following sentencing,” Rourke said.

He stressed that to say more, with the sentencing nearly two weeks away, would be to reveal too much of what investigators had learned.

“Ask me again [on] … the day after, the day of sentencing,” he said. “I will be happy to answer that question. At this point, that’s getting a little too far into the investigation at least as we sit right now.”

Those who knew the Watts family have described a seemingly happy marriage possibly beset by rising tension, and his infidelity has drawn scrutiny: Authorities have said Chris was cheating with a co-worker when he murdered 34-year-old Shanann, Bella, 4, and Celeste, 3.

A source close to the investigation previously told PEOPLE that Chris has had relationships with both men and women outside of his marriage

Image zoom Shanann Watts (center) and her daughters Shanann Watts /Facebook

Chris’ surprise plea this week — which will see him spend the rest of his life in prison in exchange for being spared a possible death penalty — put an end to his months of lies about what really happened to his family in mid-August.

First he claimed that they had disappeared on their own, with Shanann taking their daughters to an unnamed friends’ home. Later, according to his arrest affidavit, Chris claimed he killed Shanann in a “rage” at home after watching her strangle Celeste when he said he wanted to separate.

“What I can tell you most affirmatively today, by what happened in the court room, is the spotlight that he tried to shine on Shanann falsely, incorrectly and, frankly, a flat-out lie has been corrected,” Rourke said Tuesday.

“The spotlight shines directly where it belongs: on him.”

Motive is not the only still-publicly unknown fact of the case following Chris’ plea.

Asked what Chris had told authorities about his guilt — if he explained the actual timeline of the murders — Rourke said, “I don’t know the answer to that question.”

“We talked a lot about whether we would require [Chris] to come forward and give us what we hope to be a complete,, accurate and truthful statement,” Rourke continued.