It was the case that almost stumped him.
Somebody had killed at least two horses, a dog and a 20-year-old man named David Estrada during random shootings in the summer of 2005.
Detective Ronald Rock of the Tolleson Police Department was assigned the case but had few leads to go on. By December of that year, he had almost given up.
But one morning, while he was driving to work, Rock happened to turn on his car radio. He heard news that the Phoenix police were investigating two shooting deaths that appeared random and connected.
"It just sounded really strange," Rock testified Wednesday in Maricopa County Superior Court. "So when I got into work, I made a call to a detective in Phoenix that I knew."
Rock's hunch, prosecutors now say, was perhaps the earliest indication that a serial killer was loose in the Valley.
It turned out to be a good one.
Using the evidence collected by Rock during his investigation, detectives in what was later deemed the Serial Shooters case were able to connect the deaths of the animals and the young man to a string of seven other murders between May 2005 and August 2006.
Rock's testimony is some of the earliest in the trial against Dale Hausner, a 35-year-old Mesa man accused in the killings as well as in numerous other assaults.
Wednesday capped the second week of testimony in the marathon trial, which will pick up again on Monday and which attorneys have said could last through Easter.
Hausner's attorneys tried to pick apart Rock's testimony by pointing out things he neglected to investigate.
For one, the attorneys seized on earlier testimony by the owner of a dog killed in the shootings.
The owner, Carl Zolnarchik, told jurors that his tips about his dog's death were largely ignored by Rock and others investigating the cases.
At one point, Zolnarchik told them, Rock even asked him to stop e-mailing the police because he was becoming a nuisance.
Asked about those e-mails in court, Rock admitted that he indeed had ignored Zolnarchik's tips. But, the 12-year veteran said, it was because most of them seemed like mad rants from a person filled with grief.
"He was being the typical victim," Rock said. "He was very depressed over the death of his dog. ... He would send me two- or three-page e-mails and he didn't know what he was writing."
Near the end of the full day of testimony, prosecutors also began to get into the murder of Nathaniel Schoffner, the fourth killing attributed to Hausner.
Schoffner, 45, was shot while walking in central Phoenix on Nov. 11, 2005, and died two days later in the hospital.
An off-duty Phoenix police officer was the first to find him lying in the road. Authorities originally thought Schoffner had passed out drunk until they turned him over and saw a shotgun blast to his stomach.
Prosecutors have said that Hausner's brother, Jeff Hausner, helped in the shooting, though he has not been charged in it.