OAKLAND — A high-ranking Oakland city official who said he provided federal investigators key evidence that led to criminal indictments against ex-Mayor Sheng Thao and others is leaving his role to take a leadership job in another East Bay city.

The official, G. Harold Duffey, was an assistant city administrator who worked closely with Thao during her fraught tenure as the Oakland’s top elected leader, placing him the crosshairs of the former mayor’s opponents during a particularly toxic era of the city’s politics. His last day at the city was Sept. 25.

Duffey is in line to be the new city manager in Brentwood, an eastern Contra Costa County town with about 67,000 residents — roughly six times smaller than Oakland’s population. Brentwood had been without a permanent city manager since Ap

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