Mary was found dead with a gun in her hand. Then a Post-it note unravelled the lie.

On September 6, 2024, police were called to a home in South Peoria, Illinois. It was a harrowing report: a young woman had shot herself.

Officers arrived to find the body of 20-year-old Mary Halcomb on the stairwell landing. She was lying in a pool of blood, with a gun in her hand.

She suffered a single gunshot wound to her neck and was pronounced dead, according to the Peoria State Attorney's office.

But not everything was as it seemed.

Things weren't adding up. Police found a blood trail from a downstairs bedroom where the shooting had occurred up to where her body was found, as if Mary's body had been moved.

Then police made a discovery that changed everything.

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In the downstairs bedroom, police found a post-it note with a message: "I, Mary Elyce Halcomb, promise to never break Nathanial Archuleta's heart, and if I do, Nathaniel Archuleta has every right to euthanise me, vice versa, I love you."

Nathaniel was Mary's boyfriend.

When police questioned the 19-year-old, he couldn't quite get his story straight.

According to the state attorney's office, Nathaniel first claimed it was suicide. Then he said he had been cleaning his gun when it accidentally went off.

But the post-it note wasn't the only major discovery.

Police later learned that while in custody, Nathaniel told another inmate exactly what had happened.

He killed Mary, brutally shooting her dead because he caught her texting another man. Then, he staged her death to seem like a suicide.

This week, during an emotional plea hearing, Nathaniel pleaded guilty to first-degree murder and was sentenced to 45 years in prison with no eligibility for parole.

Mary has been remembered as someone who would 'do anything for anyone'. Image: Facebook/maryhalcomb.

The devastating impact of Mary's death on her family was laid bare during Monday's emotional hearing.

According to the state attorney's office, Mary's loved ones filled the courtroom as they tearfully described the emotional toll her death had caused them.

They recounted sleepless nights as nightmares met them when they closed their eyes.

Persistent heartbreak in a world without their darling girl.

They detailed their "overwhelming sense of loss, loss of sisterly moments, shared dreams, and life milestones that will never come."

Nathaniel Archuleta. Image: Peoria County Jail via Fox8 News.

Mary's mum Ashley Halcomb told the Peoria Journal Star her daughter was someone who would "do anything for anyone".

She had a "contagious" laugh and was someone who had a "heart the size of Texas."

"We did everything together," Ashley said. "She told me everything, sometimes a little bit too much, but I knew she trusted me."

Mary was studying to become a nurse and played softball. She was a "best friend" to her two younger siblings.

Now, they're left trying to understand how something so horrible could happen to their sister.

For Ashley, each day "feels like a really bad dream".

Feature image: Facebook/AshleyHalcomb.