OAKLAND -- A federal judge has appointed an investigator to look into professional misconduct claims against the monitor overseeing the Oakland Police Department.

U.S. District Judge Thelton Henderson authorized the investigation Wednesday in a written order that remains under seal, this paper has learned.

The investigation reopens the possibility that the city could oust the monitor, Robert Warshaw, who has been critical of the police department's effort to fully comply with court-ordered reforms.

Warshaw could not be reached for comment Wednesday. City officials declined to comment.

The city faces December hearings before Henderson on a potential federal takeover of the department for failing to fully implement the reforms mandated in the 2003 settlement of the Riders police misconduct case.

Earlier this month, Henderson refused the city's request "to bar all communications" with Warshaw, a retired Rochester, N.Y., police chief and deputy federal drug czar.

City attorneys had filed a motion last month disclosing that an investigation was underway into potentially damaging allegations against Warshaw.

Sources familiar with the case later said that Warshaw was accused of making sexual advances toward City Administrator Deanna Santana. That prompted attorneys seeking to put the department under federal control to demand an investigation into how that information became public and to question the timing of the city's

On Wednesday, Henderson ordered that an investigator be hired to look into the allegations. Henderson's order was filed under seal but sources say it involves the sexual harassment allegations against Warshaw.

Warshaw is responsible for evaluating the police department's progress toward reforms stemming from the Riders misconduct case.

But in the last almost 10 years, the city has failed to meet requirements mandated by the reforms, and in recent reports, Warshaw faulted the department for stagnating on several tasks that were supposed to be completed four years ago.

Henderson also this week denied a request from the Oakland police officers union to become more involved in court proceedings that could eventually lead to a federal takeover of the department.

The union currently is allowed to participate in proceedings that directly effect its collective bargaining agreement, but the union requested to be involved in all proceedings.

Matt Artz contributed to this story.