The mother-daughter pair, whose bodies were discovered by police in their Utah home on Sunday, might have died by murder-suicide nearly a month ago, the police have estimated.

Karina Clark, 41, and daughter Madison Clark, 9, were found dead on New Year’s Eve after a church member suspected that something had gone wrong at the Clarks’ house and alerted the police.

The church member had left a Christmas present on the deceased’s porch a week ago and was surprised to find that the present sat untouched even as New Year’s Day approached, which caused him to suspect that something was not right, United Police Lt. Brian Lohrke said, CBS News reported. It is believed that Karina shot her daughter three times before turning the gun on herself.

Although law enforcement agents are currently trying to determine exactly when the shooting took place, they estimated that the bodies could have been inside the house for about three weeks, without anyone noticing, depending on the time Karina last used her social media account.

Karina lived alone with her daughter and reportedly suffered from mental health issues. Despite this, the rifle that she is suspected to have used in the incident was legally registered in her name.

The motive behind the incident, which has been described as "tragic" by the police, is still unknown. "It's one of those situations where we're kind of just left in the dust right now, trying to figure out why a mother had the need to kill her nine-year-old daughter," Lohrke said. "It's a question that we may have forever."

A suicide note found at the scene said that Karina could not live in a world without her boyfriend — someone she had broken up with recently. She also shared a turbulent relationship with her other kids — Gibson Clark, 23, and Bailey Miller, 20.

Gibson told NBC affiliated KSL TV that his mother had planned the murder-suicide for days. "It was a planned death. She already had the whole house packed up. Much of it was in boxes," he said.

In a post on Facebook, Bailey wrote that she was shocked by what had happened. "My heart aches to what had to have happened that night. I have been filled with anger and sadness and have begged God to give me some sort of reason," she wrote. "My mom, despite what has been done, was a good person. She struggled from mental illness for a while but her heart was always in a good place. That is why this incident is just so shocking."

After scanning her social media activity, the police were able to find out that Karina had recently undergone gastric bypass surgery that left her in a lot of physical pain. In many of her recent posts, she wrote of her struggle to get off pain medication.

"I tried to get off my pain meds today," she wrote in one of the posts. "I didn't take one for 20 hours, but right before bed, the pain was bad enough that I knew it would be a miserable, broken sleep... so I caved. Perhaps I can just take them in the eve's for a few more days before quitting them completely. I mean... I've only been out of surgery for 6 days, right? I just don't like being dependent on anything."