An Orange County Sheriff’s deputy labeled a liar by a judge is now refusing to testify as a key witness in cases involving the Mexican Mafia, forcing prosecutors to offer deals or rethink charges.

The District Attorney’s Office confirmed Friday that Deputy Seth Tunstall, a statewide expert on the Mexican Mafia, notified prosecutors through his lawyer that he will not testify as planned.

Defense attorneys in some of the affected cases said prosecutors told them Tunstall would invoke the 5th Amendment – his right against self-incrimination – if called to testify.

Tunstall’s refusal is the latest turn in the nationally watched informant crisis that has dogged District Attorney Tony Rackauckas, with allegations that police and prosecutors routinely withheld evidence gathered by an illegal network of jailhouse snitches.

Sheriff Sandra Hutchens did not comment Friday, saying through a spokesman that she hasn’t yet talked to Tunstall.

Tunstall, a 14-year veteran, was unavailable for comment, the department said.

Susan Kang Schroeder, chief of staff for Rackauckas, said Tunstall’s refusal to testify would affect only a handful of cases.

“It’s not unusual for us to have cases that can’t proceed because we lose a witness,” Schroeder said.

During a March hearing on the informant allegations, Superior Court Judge Thomas Goethals ruled that Tunstall and Deputy Ben Garcia “either intentionally lied or willfully withheld material evidence” from the court. The judge made his comments during a hearing on defense motions involving the death penalty case against Scott Dekraai, who admitted killing eight people at a Seal Beach salon.

Defense attorneys in other cases involving Tunstall said they were preparing to use Goethals’ ruling – and other evidence – to attack the deputy’s credibility.

“In our case, he had changed his testimony in material ways, he flipped on material issues,” said Doreen Boxer, who represented inmate Estavan Cardoso in a jailhouse assault. In exchange for Cardoso’s guilty plea, he was released from jail with four years’ credit for time served.

So far, seven defendants charged in two jailhouse attacks have been offered plea deals, some on the day they were to begin trial. Charges that the defendants committed the crime to enhance their standing with the Mexican Mafia were dropped.

“There was no offer on the table, then on the day of trial we received an offer,” Boxer said. “We let it be known we were going to be very aggressive and effective in defending our client against Mr. Tunstall.”

In another assault case, Tunstall’s testimony before the grand jury was pivotal in getting an indictment against inmate Luz Gallardo, said his attorney, David Swanson.

Tunstall was the investigating officer in the case against Gallardo. Tunstall also worked in the jail as one of three “special handlers” who managed and cultivated informants.

Senior Deputy D.A. Dave Porter, the prosecutor in the recently settled cases involving Tunstall, said he didn’t blame the deputy for keeping mum.

“To expect him to come forward and make statements that could be used against him, we couldn’t expect that of anybody,” Porter said.

Contact the writer: tsaavedra@ocregister.com