How a small piece of a bathroom door lock helped solve the murder of a Minnesota nurse

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

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+. MATTHEW ECKER: No.

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