For years, relatives and friends of 25-year-old Heidi Firkus have been puzzled by her death in 2010 at the home she shared with her husband, Nick, in Saint Paul, Minnesota.

Heidi's murder was unsolved until investigator Nichole Sipes of the Saint Paul Police Department was assigned the case.

"It never felt right. The story never made sense to me," Sipes tells "48 Hours" contributor Jamie Yuccas in "Death at the Front Door: Who Shot Heidi Firkus?" airing Saturday, Dec. 2 at 10/9c on CBS and streaming on Paramount+

When Sipes took over the investigation in 2019, she reviewed the entire case file and listened to the 911 calls made on the day of Heidi's death.

Home Invasion Tragedy

On a fateful night, a terrifying incident unfolded in a quiet suburban neighborhood. Heidi, a resident of the area, made a frantic call to 911, reporting an attempted break-in at her home. However, before she could provide further details, a loud noise and a scream were heard, leaving the operator in shock.

Moments later, Heidi's husband, Nick, called 911 from her phone, revealing the horrifying truth. He informed the dispatcher that someone had broken into their house and shot both him and his wife.

When emergency responders arrived at the scene, they tragically pronounced Heidi dead. Nick, on the other hand, was fortunate enough to survive the attack, although he sustained a gunshot wound to his leg. He was immediately rushed to the hospital for treatment.

Several hours later, Nick arrived at the police station to provide a statement. He recounted the events leading up to the incident, explaining that he had woken up in the early hours to get a drink of water. It was during this time that he heard suspicious noises coming from the front door.

Acting quickly, Nick retrieved his shotgun from the closet and woke Heidi, instructing her to call 911. Together, they made their way downstairs, intending to escape. However, as they reached the front door, the intruder managed to enter, sparking a violent struggle.

In a shocking revelation, Nick confessed to investigators that he accidentally fired his shotgun, hitting Heidi in the back as she was running away. In the chaos, the intruder retaliated by shooting Nick in the leg before fleeing the scene.

During the investigation of the crime scene and interviews with neighbors, it was revealed that no one saw anyone fleeing from the Firkus house. However, Brendan O'Connor, who was taking care of kittens next door, informed the police that he heard a muffled argument followed by gunshots and a voice pleading, "you shot her, you shot me, please, please, no..."

Further inquiry into the couple's financial situation showed that Nick and Heidi were behind on bills, had lost their home to foreclosure, and were scheduled to move out within 24 hours. Detective Sipes stated, "There was no indication of any communication between them regarding the extent of their financial issues. It appears that Nick intentionally concealed their financial troubles from Heidi."

"I believe he was concerned about the shame of what he had done, how it would look... that he couldn't be honest with her," said Sipes.

It was also discovered that Nick had remarried after Heidi's death and subsequently divorced. During an interview with his ex-wife, Rachel Firkus, she revealed that Nick had not been paying their bills as well. Rachel described this as a recurring pattern, similar to the financial situation they faced during their marriage. "He was definitely repeating the same behaviors he exhibited with Heidi," Rachel told "48 Hours." She even provided audio recordings of her conversations with Nick, which she claims contained his lies about their financial problems.

Man Found Guilty of Double Murder

In a shocking turn of events, Nick Firkus has been convicted of the brutal murders of Heidi and another unidentified victim. The trial, which began on January 27, 2023, concluded with the jury finding Firkus guilty on two counts of murder.

Prosecutors had presented compelling evidence, asserting that Firkus had no alibi for the time of the crime and that there was no evidence of an intruder at the scene. They also alleged that Firkus shot himself in the leg in an attempt to cover up his heinous acts.

Despite the overwhelming evidence against him, Firkus maintained his innocence throughout the trial and continued to do so during his sentencing. "I do maintain and will maintain to my dying breath my innocence of this crime," he declared defiantly.

The presiding judge, however, was unmoved by Firkus' claims and handed down a life sentence without the possibility of parole. Firkus is currently in the process of appealing his conviction.