By Curtis Skinner

SAN FRANCISCO (Reuters) - The troubled police force in Oakland, California, saw its third police chief resign in just over a week on Friday as investigations into sexual misconduct and racist text messages continued to roil the department.

Oakland Mayor Libby Schaaf told a Friday evening news conference that acting police chief Paul Figueroa resigned his duties, a mere two days after he was appointed to replace Ben Fairow as the department's top cop.

"I want to assure the citizens of Oakland that we are hell-bent on rooting out this disgusting culture," Schaaf said angrily, calling the environment in the department "toxic" and "macho."

Schaaf has declined to provide details about the ongoing investigation into allegations of sexual misconduct against the department, claiming that releasing information could impede the probe and possible charges.

The local East Bay Express newspaper reported last Friday that as many as 21 officers from the Oakland police department and other area law enforcement agencies had sex with a teenage sex worker, including some while she was underage.

The newspaper based its report on interviews with the woman, elected officials, Oakland police sources as well as documents. Other media outlets have since published similar accounts.

Schaaf did acknowledge on Friday that an unrelated investigation was underway into the sharing of racist text messages by some black officers. Local broadcaster NBC Bay Area reported some of the messages contained racial slurs and images of the Ku Klux Klan.

Schaaf said she would not appoint another acting chief and command staff would instead report to City Administrator Sabrina Landreth.

"I feel that this is an appropriate time to place civilian oversight over this police department," she said.

Former Oakland Police Chief Sean Whent, who had headed the department since May 2013 and was heralded by Schaaf for recent declines in shootings and murders, resigned last Thursday. Schaaf declined to elaborate on the move aside from saying Whent made a "personal choice."

Schaaf replaced Whent with Ben Fairow, but changed course and removed him on Wednesday, saying only that she received "information" that made her question whether he could lead the department.

The news comes just days after the release of a Stanford University study on the department, which found that African American men were four times more likely to be searched during police traffic stops than whites and were more likely to be handcuffed even if they were not arrested.

(Reporting by Curtis Skinner in San Francisco; Editing by Richard Pullin)