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 died from a single shotgun blast to her back that her husband said she suffered as he wrestled with an intruder. Nick Firkus told police a man burst through their front door in the dawn hours of April 25. Moments earlier, at his request, Heidi had called 911 to report an intruder was breaking into their home. 

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?" now 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.

"Someone's trying to break into my home," Heidi told a 911 operator. Near the end of her call, a loud noise is heard, followed by a scream from Heidi. Approximately 65 seconds later, Heidi's husband called 911 from her phone. "Somebody just broke in my house and shot me and my wife …"

Sipes learned when first responders arrived at the scene, Heidi was pronounced dead. Nick was transported to the hospital, where he was treated for a gunshot wound to his leg. A few hours later, Nick arrived at the police station for an interview.  He told investigators he awoke in the early hours to get a drink of water from the bathroom and said he heard someone fiddling with the front door. Nick then said he grabbed his shotgun from the closet, woke Heidi and told her to call 911. Nick said he rushed Heidi down the stairs so they could escape but as they passed the front door, the intruder entered and that struggle ensued.

"So my finger slipped onto the trigger … she was running away, so I definitely hit her in the back," Nick told investigators. That's when the intruder shot him in the leg, before fleeing, he said.

As investigators examined the crime scene and canvassed the neighborhood, neighbors said they did not see anyone running away from the Firkus house. However, Brendan O'Connor, who was house sitting kittens next door, told police he heard a muffled argument coming from Nick and Heidi's house followed by gunshots and a voice yelling, "you shot her, you shot me, please, please, no ..."

Nick also told investigators he and Heidi were behind on bills, had lost their home to foreclosure and were scheduled to move out in 24 hours. "There was no communication between the two of them to indicate that she had any idea of the depth of their financial issues," Sipes said. The investigator believes Nick was deliberately hiding 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 come clean with her," Sipes said.

Sipes learned Nick had remarried after Heidi's death and was now divorced. When Sipes interviewed his ex-wife, Rachel Firkus, she told her Nick had not been paying their bills. A secret, she says, he kept from her. 

Rachel says it was a pattern that seemed like the financial situation years earlier with Heidi. "He was definitely repeating the same things he did with Heidi with me," Rachel told "48 Hours." Rachel even shared audio recordings of her conversations with Nick and what she says were his lies regarding their financial problems.

In 2021, prosecutors said they had enough evidence to charge Nick Firkus with Heidi's murder. They said there was no intruder at the home that day and Nick shot himself in the leg after he murdered Heidi.

On Jan. 27, 2023, Nick Firkus went on trial. He was found guilty on two counts of murder. A few months later, at Nick's sentencing, he refused to accept guilt: "I do maintain and will maintain to my dying breath my innocence of this crime." The judge gave Nick a life sentence without parole. Firkus is appealing his conviction.

In:
• 48 Hours
• Minnesota
• Murder