OAKLAND, California: An investigator appointed by a US federal judge is examining whether campaign contributions affected how the city of Oakland, California, chose its lawyers for police disciplinary cases, according to a source briefed on the probe.

The investigation started after an arbitrator reinstated an Oakland police officer who was heavily criticized for firing tear gas at a crowd of protesters during Occupy protests in 2011. That followed a series of other cases in which arbitrators overruled disciplinary actions for Oakland police.

Disciplining problem officers is viewed by police reform advocates as a crucial tool for maintaining community trust — something that is taking on heightened importance as national protests smolder over aggressive police tactics in Ferguson, Missouri, and New York City.

For the tear gas arbitration, Oakland City Attorney Barbara Parker hired Stephen Roberts, a partner at large California-based law firm Nossaman. In 2011, Roberts and two other Nossaman attorneys donated $700 each to Parker’s campaign, the maximum allowed by law, according to campaign finance records. Four other Nossaman attorneys donated a total of $950 to Parker’s campaign.

It is not clear on which contributions the investigation is focusing, nor when the probe will conclude. The investigator, San Francisco attorney Edward Swanson, declined to comment.

Parker said it is “categorically untrue” that contributions play any role in how the city hires lawyers. Attorneys who never donated money have been retained by the city, she said, while other contributors have not gotten city business.

“There’s obviously no quid pro quo here,” she said. Roberts did not respond to requests for comment.

In Oakland, officers can appeal any discipline meted out by the city to an independent arbitrator.

Police reform advocates generally view arbitration as a stumbling block for cities trying to hold cops accountable.

“It tells the conscientious, rule-abiding officers it doesn’t make any difference, you can screw up, and you’ll be able to keep working,” said Mark Iris, a lecturer at Northwestern University and former executive director of Chicago’s police-oversight board.

However, attorneys who represent officers see arbitration as a critical tool to protect public employees from unfair discipline that’s often fueled by political pressure.

Oakland police have been under court oversight since 2003, when the city agreed to a raft of reforms codified in a consent decree. In ordering the arbitration probe earlier this year, US District Judge Thelton Henderson wrote “imposition of discipline is meaningless if it is not final.”