Husband Sought Divorce Before Texas Mom Killed Their Daughters, Telling Her, 'You Have Ruined My Last Birthday'

On the day Texas mom Christy Sheats fatally shot her two daughters, her husband thought she had called a family meeting at their home, on his 45th birthday, to discuss his decision to file for divorce.

"You have ruined my last birthday," Fort Bend County Sheriff Troy Nehls tells PEOPLE exclusively, recalling what Jason told investigators he said to his wife in announcing his intention to leave her, prior to the violent outburst Friday.

In an in-depth interview with authorities Tuesday, Nehls says Jason helped them reconstruct what happened, and why.

That afternoon, with the family of four gathered in their Katy, Texas, living room, Christy Sheats, 42, produced a .38-caliber handgun inherited from her grandfather and opened fire on the couple's two daughters, Taylor, 22, and Madison, 17.

Both daughters died after fleeing out the front door with Jason. Authorities responding to 911 calls said they shot Christy dead when she refused to drop her weapon after the officers witnessed her last shot at Taylor and then approached them.

Jason told investigators his wife suffered from depression and anxiety, was taking several medications to cope with each and had made three suicide attempts. He also told them that over the past four years, as the couple struggled with marital problems, she'd been treated at three private mental health facilities.

Jason said he'd been out of the house for part of that day with one of his daughters, but he had been in touch with Christy by text. He returned home to find his wife sitting in the backyard, where he "told Christy he would be filing for divorce," Nehls tells PEOPLE.

Jason then walked to a another part of the home, but Christy announced that she wanted to call a family meeting, Nehls says. Jason thought it was to tell the girls about the divorce – he never expected she would be violent.

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Jason and the daughters walked from a computer room to the living room, sitting down with Christy, Nehls says. "She immediately pulled the weapon out and pointed it at Mr. Sheats and wanted to blame him, 'You made me do this,' " the sheriff says, recalling what Jason told authorities.

"Having all the opportunity to pull that trigger, she pointed that gun directly at Mr. Sheats," Nehls says.

When Jason was asked why he thought Christy didn't shoot him, he told investigators "he felt she wanted him to suffer," because Christy knew how much he loved his daughters and how much they loved him.

"This was a way for her to accomplish what she set out to do," the sheriff says.

"He's very lucky to be alive."