In the early morning hours of Dec. 16, 2022, St. Paul, Minnesota, homicide detectives Abby DeSanto and Jennifer O'Donnell were called to a downtown apartment building to investigate a reported suicide. A 32-year-old woman named Alexandra Pennig had been found dead in her bathroom with a single gunshot wound to the head.

For the detectives, what really happened to Pennig is something that still haunts them to this day. And it's the question at the center of "The Strange Shooting of Alex Pennig," reported by "48 Hours" contributor Natalie Morales airing Saturday, Oct. 26 at 10/9c on CBS and streaming on Paramount+.

When detectives DeSanto and O'Donnell arrived at the apartment, they found out Pennig had not been alone at the time of her death. A man named Matthew Ecker was also there. Ecker and Pennig were both nurses and had met two years earlier when they worked at the same clinic. Ecker told first responders the gun was his, and that Pennig had grabbed it, locked herself in the bathroom, and then fired the shot. "I thought everything was fine," he said. "And then she just grabbed the gun." Ecker told first responders that after he heard the shot he immediately broke open the bathroom door: "I tried to do what I could. And then I washed my hands … That's why I don't have anything on my hands." Ecker said he then called 911. But it was too late. He said he didn't know why Pennig would do this.

In Pennig's apartment, there was alcohol and six bottles of prescription medication, including antidepressants, all prescribed to Pennig. For the detectives, it suggested Alex might have been depressed, and they wondered if Ecker's story that she took her own life was true.

But they also noticed something that seemed to contradict Ecker's story. He had said he washed his hands in the bathroom sink before calling 911, but DeSanto recalled the first responders told her the sink was dry. "The sink was dry. If he had said, you know, he called the police right away, that sink probably would've been still wet," DeSanto explained, "but it was very dry in there."

When O'Donnell looked into Pennig's background, she learned from Alex's parents that Alex had struggled in the past with depression and addiction. "I had asked, um, if she had been suicidal in the past, um, and dad said, she had, um, tried, uh, to overdose before," said O'Donnell. According to Alex's father, Jim Pennig, several years prior, Alex had taken a handful of pills "and then had told her mom that she was attempting suicide." After that, Alex's parents told the detectives they sent her to rehab, and she eventually got clean. Despite her past struggles, Alex's parents told O'Donnell they had just seen her at Thanksgiving. And her mom, Mary Jo Pennig, had just talked to her that evening. "She was doing well," she said. For them, the idea that their daughter had died by suicide did not make sense. "Knowing your kid, it didn't fit," Mary Jo Pennig said.

Since Ecker was the last person to see Alex Pennig alive, the detectives zeroed in on him. "He's the only one that can tell us what happened. He was the only one that was there," said O'Donnell. They questioned Ecker about what had happened that night. He said he and Alex Pennig had gone out to several local bars, and when they arrived back at her place, everything was fine: "We were laughing on the way home," said Ecker. DeSanto asked him if, once they got into the apartment, they had gotten into a fight. Ecker said they did not.

DET. ABBY DESANTO: You guys weren't arguing or anything?

MATTHEW ECKER: No.

DET. ABBY DESANTO: There's no fight with you two?

MATTHEW ECKER: Not between us.

For hours, Ecker continued to say Pennig had locked herself in the bathroom, fired the shot and then he broke open the door to try and help her: "That gun went off behind a closed door … I did not shoot her."

But the detectives had their doubts. Then they got a call from the forensic unit that was still processing the scene. And according to O'Donnell, what they found changed everything. "Once Alex was moved, they found underneath where Alex had been laying was a round metal piece," she said. It was the shape of a ring, and about the size of a quarter. O'Donnell said it was part of the lock from the bathroom door, and the fact that it had been discovered under Pennig was key. "For us, it meant that the door was forced open before she was shot."

The detectives felt the discovery of the metal ring proved Ecker had lied and had not broken the door open after he heard the shot. The detectives suspected Pennig and Ecker had argued and that she had locked the bathroom door to get away from him. Then Ecker broke open the door, the metal part broke off and fell to the ground, and then he shot Pennig and she landed on top of it.

Ecker was charged with second-degree murder. In February 2024 he was convicted and later sentenced to 30 years. He is appealing his conviction.