A crime scene investigator is gunned down in broad daylight. The harrowing scene is captured on surveillance video. Who pulled the trigger? "48 Hours" correspondent Erin Moriarty reports.

Angela Brosso, 21, and Melanie Bernas, 17, were separately killed while riding their bikes along the Phoenix Canal.

These murders became widely known as the Phoenix Canal Murders," says Briana Whitney, the true-crime reporter for CBS affiliate KPHO-TV in Phoenix.

Investigators got a break 21 years after the murders when, thanks to DNA and genetic genealogy, they zeroed in on 42-year-old Bryan Patrick Miller, a divorced father raising his teenage daughter. Investigators soon found out that Miller had an alter ego. He was a local celebrity known for participating in parades and festivals as the "Zombie Hunter," a zombie-slaying comic book hero armed with a larger-than-life Gatling gun.

Detectives just needed a sample of Miller's DNA to make the case or eliminate him as a suspect. That quest took them to a local Chili's.