Little did they know.
The 52-year-old Californian confessed to pulling the trigger in that March killing, the Lawrence County, Alabama, Sheriff's Office announced Thursday. He didn't stop there: Martinez also admitted killing more than 30 men in all, much like he did Jose Ruiz.
"Killing people doesn't seem to affect him," sheriff's Capt. Tim McWhorder said.
As of Thursday, authorities had identified 13 violent deaths linked to Martinez since his admission. At least 10 of those occurred in California, according to McWhorder.
He's also been tied -- by a DNA match to a cigarette butt found inside a victim's truck -- to a 2006 double homicide in Ocala, Florida, the Marion County, Florida, Sheriff's Office announced Wednesday.
Martinez explained that his record of violence has to do with his job as an enforcer for Mexican drug cartels. A U.S. citizen, he'd be called when someone hadn't lived up to their obligations.
As he told investigators, "I'm the guy that pays you a visit if you don't pay."
Opinion: Looking into the minds of killers
The killing of Jose Ruiz, though, was personal, authorities say.
As McWhorder explained, the two had entered into "some type of business arrangement" in Alabama, where Ruiz lived in Decatur and Martinez had family.
While working together, Ruiz bad-mouthed the girlfriend of another man, Jamie Romero, calling her names and "a bad woman."
Apparently, he didn't know that Romero's girlfriend was also Martinez's daughter.
That happened in January. Stewing on it for two months, Martinez returned to Alabama having "made up his mind he was going to kill him," McWhorder said.
That's what authorities now believe happened. All three men -- Ruiz, Romero and Martinez -- drove in Romero's truck to the outskirts of Bankhead National Forest.
Ruiz didn't make it out alive.
Authorities found evidence linking Romero to the death and arrested him a few days later. Romero said that earlier on the day of Ruiz's death, he'd been with Martinez. When investigators questioned him, Martinez admitted he'd been with Romero that day but denied having anything to do with that killing, according to the Lawrence County Sheriff's Office.
As rumors swirled, and the investigation went forward, authorities came to believe Martinez had lied.
An arrest warrant was issued for him, and he was picked up in Yuma, Arizona, just over the border from Mexico, where McWhorder said he'd been visiting family. The sheriff's captain said authorities now believe "that Romero may not have known that the murder was going to take place."
On June 3, Martinez was brought back to Alabama and charged with felony murder in Ruiz's death. That's where he was Wednesday, in Lawrence County Jail, after a judge ordered him held without bail.
More charges, and more answers, may be coming as authorities probe his possible connection to more killings from coast to coast.