A Sonoma County detective testified Tuesday that he tipped Santa Rosa gang members to a threat on their lives hours before they went looking for revenge and shot the wrong man.

Sheriff's Det. Brandon Cutting took the stand in the second day of testimony in the murder trial of Christopher "Spider" Mancinas, 30, who's charged along with three others with the mistaken-identity slaying of Vallejo musician Dewey Tucker, 24.

Tucker, who had no gang ties, was a bass player for hip hop singer Lauryn Hill.

Cutting testified that on Jan. 12, 2010, he seized a letter written by a Sonoma County inmate calling for "hits" on four gang rivals and a girlfriend suspected of witnessing another killing.

Fearing there could be violence, Cutting said he went to the homes of at least two of the men mentioned in the letter and warned them they could be in danger, he testified.

"We had an obligation to notify the people," Cutting testified.

One of the men was Hector Barragan, 29, an acknowledged leader of the Angelino Heights sureno gang that was embroiled in a turf war with other surenos in southwest Santa Rosa.

Prosecutors believe once Barragan learned of the threat, he assembled a small group including Mancinas, Javier Carreon-Lopez, 21, and Raul Vega, 20, and traveled to a Vallejo apartment complex in search of retribution.

They believed a man would be there who had killed one of their own several months earlier in an escalation of fighting over control of Southwest Community Park.

When the armed men arrived outside the apartment building that night, they saw a car resembling one driven by their rival pulling out of the driveway, prosecutors said.

They followed it onto Interstate 80, pulled alongside and fired at it, killing Tucker, who they could not see because it was dark.

The case was unsolved for about 10 months until Mancinas was arrested on drug charges and told police. Although he had ties to the Mexican mafia, he was a longtime police informant and had friends on both sides of the rivalry, prosecutors said.

Barragan, his older brother and another sureno leader were also mentioned in the letter along with a woman who was suspected of witnessing the 2009 killing of a member of Barragan's gang.

The letter said they all needed to be "taken out," Cutting said.

Cutting testified that after talking to other violent crimes detectives, it was decided that the gang members should be warned, through Barragan. Cutting talked to him at his house.

"What I discussed with him was, &‘Are you going to be OK?' and &‘Do you need anything from us?" Cutting testified. "He said he did not."

Although the case involves three counties, it is being tried in Sonoma County by an earlier agreement. Barragan, Vega and Carreon-Lopez also will be tried in Sonoma County later this year.

All are charged with murder, gang participation, shooting into a vehicle and conspiracy.

Vega is thought to be the gunman in the Tucker case. He's also charged with murder in a separate slaying at Kawana Springs School in Santa Rosa last year.