A former Contra Costa County sheriff's deputy who allegedly helped a private investigator with "dirty DUI" arrests has been indicted by a federal grand jury.

Stephen Tanabe, 48, of Alamo, was indicted Thursday on four felony counts of conspiracy to extort under color of official right and aiding and abetting, in connection with the drunken-driving arrests. He is expected to surrender to authorities before a scheduled court appearance Monday in Oakland, attorneys said.

Tanabe is the fourth former law enforcement officer with ties to former private eye Christopher Butler, 50, to face federal charges. Butler pleaded not guilty to federal drug and conspiracy charges earlier this year.

In July, two former Richmond police officers who had hired Butler to carry out a retaliatory sting against two young men who once worked for them were indicted on gun charges.

In August, Butler and Norman Wielsch, 50, a former state Justice Department agent who commanded an antidrug task force in Contra Costa County, were indicted on charges connected to the alleged theft and sale of drugs.

Authorities said Wielsch had pilfered the drugs from evidence lockers.

Tanabe pleaded not guilty this year in state court to bribery and conspiracy charges connected to two drunken-driving arrests he made in Danville in January.

Prosecutors said Butler had paid Tanabe to pull over and arrest men the detective was hired to go after. Butler was working for women in divorce cases who wanted to saddle their ex-husbands with criminal records to help them win custody battles, authorities said.

In an affidavit, a cadet-in-training who was riding with Tanabe in January said the officer used the term "dirty DUI" before he stopped a man Butler had hired him to arrest.

In a confession written by Butler and obtained by The Chronicle, the former investigator said he had supplied $200 worth of cocaine to Tanabe as payment for his involvement in one DUI stop.

Tanabe's attorney in the federal case did not respond to a phone call.

Dan Russo, the attorney who is representing Tanabe in state court, said, "It's obvious the feds are buying into Butler's perjurious lies."

Russo says Tanabe didn't know Butler was setting up the men he was arresting for drunken driving, and denies the former deputy ever took a bribe.