A Santa Rosa man who allegedly helped plan the kidnapping and murder of a witness in a Sonoma County gang trial was arrested Tuesday morning, authorities said.

The arrest comes nine months after members of the Asian Boyz street gang allegedly drove 24-year-old Vutha Au of Santa Rosa to a beach near Jenner and shot him nine times.

Boonlack Chanpheng, 27, of Santa Rosa was arrested after leaving his house at about 9:30 a.m., sheriff's officials said.

Sgt. Dave Thompson said Chanpheng is a member of the Asian Boyz, who allegedly helped plan the crime against Au, and "that puts him as a conspirator to murder and kidnapping."

Authorities believe Au was killed so he couldn't testify on behalf of his brother, 22-year-old Terry Au, who has alleged he also was kidnapped and tortured by the same gang when he decided to stop running drugs for them.

Quentin Russell, 25; Preston Khaoone, 22; Tyrone Tay, 26; and brothers David Prak, 19, and Sarith Prak, 22, remain held without bail at Sonoma County Jail on murder charges.

Thompson said Chanpheng was not at the scene of the killing.

"He helped orchestrate or plan this out, or was in the planning stages," Thompson said. "That's what detectives were able to uncover."

Thompson said detectives from the sheriff's violent-crimes unit pieced together Chanpheng's alleged involvement through searches of phone and bank records, as well as other "investigative avenues."

"That's just the way these investigations kind of unfold in some cases," he said.

District Attorney Stephen Passalacqua said in a statement Tuesday that his office worked closely with sheriff's investigators.

Officials said both Vutha and his brother Terry had been placed in the California Witness Relocation program and agreed to remain out of the area. But Vutha Au had returned to Sonoma County.

Chanpheng faces charges of kidnapping, murder and participating in a criminal street gang.

Chanpheng is scheduled to be arraigned on murder charges Friday, and a preliminary hearing for Russell, Khaoone, Tay and the Prak brothers is scheduled for Jan. 20.

You can reach Staff Writer Martin Espinoza at 521-5213 or martin.espinoza@pressdemocrat.com and Staff Writer Laura Norton at 521-5220 or laura.norton@pressdemocrat