A San Pablo gang member shot and killed two young men last weekend on a busy footpath in downtown Pinole because one of them was wearing a color claimed by a rival street gang, authorities said Tuesday.

But the victims, 18-year-old David Gregory of Hercules and 21-year-old Darren Kretchmar of Pinole, weren't in a gang. Gregory was simply wearing a red shirt, authorities said.

"I can't imagine a more callous and trivial motive," said Contra Costa County prosecutor Harold Jewett, who heads his office's homicide team.

Jewett filed two murder charges Tuesday against Daniel Ruiz, 25, which make him eligible for the death penalty. He is scheduled to be arraigned today in Martinez.

Jewett said Ruiz, who has a criminal record and extensive gang tattoos, is a member of the Sureño gang, which claims the color blue and is in a long, bloody rivalry with the Norteños, who claim red.

Just before 6:45 p.m. Saturday, Ruiz and a young woman, who were arm in arm, walked up to the victims and two other friends as they drank beer under a wooden train trestle just outside Fernandez Park, a city gathering place. Kretchmar lived nearby.

Ruiz said to Gregory, "What's up, homey?" and then almost immediately opened fire with a .380-caliber handgun, Jewett said. He shot Gregory in the head from close range, then shot Kretchmar twice in the head, and finally shot 19-year-old Richard Male of Pinole in the upper body, puncturing a lung, Jewett said.

Male and a young woman survived the shooting. Ruiz, who is also charged with attempted murder for shooting Male, was soon arrested while trying to hide in a nearby backyard. The woman who was with him has not been located, nor has the gun, police said.

Ruiz had been carrying a photograph of a Sureño member who had been killed recently and appears to have killed Gregory and Kretchmar -- who were strangers -- in retaliation, Jewett said.

"I'm pissed. I'm furious. I can't believe it," Kretchmar's sister, Diana Basham, said Tuesday. "He walked through the park and randomly chose who he was going to kill that day."

Sureños, or southerners, and Norteños, or northerners, are offshoots and loose affiliates of two Latino prison gangs that have been at war since the late 1960s. Each of the victims in Pinole was white.

There are few statistics on the number of crimes committed by Bay Area Norteños and Sureños. But law-enforcement officers say they have seen a trend toward younger members and an increase in the intensity of violence.