Cold case Detective Clark Schwartzkopf was working a double murder case from the early 1990s and had arranged to meet a person of interest at a Chili's restaurant in Phoenix, Arizona. The idea was to get the possible suspect's DNA so forensic investigators could determine if it matched male DNA found on the bodies of the two female victims, Angela Brosso and Melanie Bernas.

Those unsolved murders had become known as the canal murders because both women were attacked while taking bike rides along the city's distinctive canal paths.

It was Jan. 2, 2015, and Schwartzkopf was meeting with a guy named Bryan Patrick Miller. The detective had learned that Miller was something of a local celebrity. He liked to attend popular zombie walks and other festivals in Phoenix dressed as a character known as the Zombie Hunter, and fans and police officers posed with him.

When he was in character, Miller wore a homemade costume with goggles and a menacing mask and carried a fake Gatling gun. He also drove and tricked out an old Crown Victoria police car, splashed it with fake blood and put the name Zombie Hunter on the back. He also often put a ghoulish mannequin in the back seat behind bars.

The day they met for dinner, Miller had driven his distinctive car to Chili's. That certainly caught Schwartkopf's attention, but the detective still didn't believe Bryan Miller was the canal killer. One of the reasons he was a potential suspect was because a genetic genealogist had matched the name Miller to the crime scene DNA.

But Schwartzkopf remained skeptical that this 42-year-old divorced dad raising a teenage daughter alone and working at an Amazon warehouse could be the killer.

Schwartzkopf told "48 Hours" correspondent Peter Van Sant that he essentially wanted to cross this Miller off his list. "I was really more about just getting his DNA," Schwartzkopf said, "clearing him and moving on because my conversation with him, he was the last person I ever thought would be responsible for this. He was mild-mannered."

Van Sant reports on the case in "Unmasking the Zombie Hunter," airing Saturday, Oct. 21 at 10/9c on CBS and streaming on Paramount+.

The detective's doubts were reinforced as he observed the way Bryan Miller treated his teenage daughter whom he had brought along for the meal at Chili's. "He had a good rapport with his daughter."

Van Sant asked, "Was he physically imposing? He looked like a guy who could overpower people?" 

The canal killer had ambushed Brosso and Bernas while they were each out on nighttime bicycle rides. Somehow, the killer had stopped them, and then stabbed and sexually assaulted them. The killings were particularly vicious and Brosso had been beheaded.

"He's a bigger guy, but he's more soft," the detective said. "Big enough to where he could certainly overpower women, but not someone you'd be really scared of on the street."

Schwartzkopf had arranged to be seated in a quiet section of Chili's. Undercover detectives watched workers take the silverware and plates out of the dishwasher and then detectives placed them on the table to ensure they would not be contaminated, Schwartzkopf told "48 Hours."

Miller ordered a burger and a glass of water. "He swallows his hamburger, in like, five bites, Schwartzkopf said. "Won't take a drink of his water. And I'm sitting there going, 'Are you sure you…don't want…something else to drink? You just got water.' 'No, no, I'm good, I'm good, I'm good.'" 

But then to Schwartkopf's relief, Miller finally took a sip of water: "That's when I knew that, OK, now we've at least got his DNA."

As soon as Miller walked out, undercover detectives secured the glass Miller had used. Miller then gave Schwartzkopf a quick tour of his Zombie Hunter car before he left.

Eleven days later, the detective got a visit from the head of the forensic lab. Schwartzkopf told Van Sant what happened. "She leans down to me, she goes, 'It's him.' I go 'What?' She goes, 'Bryan Miller, it's him.' … Well, the blood rushed from my head. … I kind of sat back and I went, 'You've gotta be kidding.'"

Miller was arrested immediately and denied he had killed anyone. The case took almost eight years to get to trial, but Miller was convicted of murdering Brosso and Bernas and received the death penalty in June 2023.

Under Arizona law, Miller will receive an automatic appeal.