The body of Ruben Leal was found in a kiddie pool filled with ice in the backyard of a Santa Ana home in 1998.
The 20-year-old had been shot in the head with a high-powered assault rifle.
The residents told police Leal committed suicide, but investigators always suspected rival gang member Ramon Alvarez was the shooter. On Wednesday, after a second murder trial, a jury agreed.
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After deliberating about 10 hours, the jury in Santa Ana found 37-year-old Alvarez guilty of second-degree murder. He now faces up to 45 years to life in state prison at his July 6 sentencing.
A judge earlier this year declared a mistrial when the first jury could not reach a unanimous verdict.
The case against Alvarez was aided by a jailhouse informant who years earlier could not come to agreement with authorities to testify. Dying of liver cancer, the man later decided he wanted to do something good and agreed to go on the record fingering Alvarez, who was arrested in 2010.