PITTSBURGH - Pittsburgh Police Chief Nate Harper stepped down Wednesday at the behest of the city's mayor, who earlier in the day met with federal investigators amid a probe of police business.

Harper's resignation comes a week after the FBI gathered boxes of records from police headquarters and just days after City Controller Michael Lamb announced an audit of the force's special events office.

Mayor Luke Ravenstahl told reporters he spoke with Harper over the phone but did not elaborate on their discussion.

"I'm not going to talk to you about the chief's reaction," he said. "This is a sad day."

Harper has denied any wrongdoing. Efforts to reach him by phone and email by The Associated Press were not immediately successful.

Ravenstahl ordered a review of the police bureau on Feb. 8 after learning that Harper partnered with four subordinates to form a private security firm - including a man he promoted to commander. But that review and the searches by federal agents were just the latest in a series of events that drew unwanted attention to the 900-officer department.

Last year a former friend of Harper's was charged in connection with a bribery scheme involving a contract to install radios and computers in police cars. In January, five off-duty police officers shot at a car on a crowded street after a chase in an incident that remains under investigation. And in early February a former cashier for the police records office was charged with stealing more than $15,000.

Harper became chief in October 2006 after working his way up through the ranks. He had been with the police force for more than 30 years. Regina McDonald, the department's assistant chief, will serve as acting chief in the interim.

Ravenstahl, who announced his intention to seek re-election on Tuesday night, said federal investigators told him he's not a target of their investigation.

On Tuesday, Ravenstahl said officials would investigate several police-related bank accounts to determine who controlled them and how the money was spent. That probe involves several accounts at the Greater Pittsburgh Police Federal Credit Union, from which the FBI also seized documents last week.

The mayor has said he doesn't know why money from one account paid to rent condos downtown for officials whose homes were threatened by Group of 20 economic summit protesters in 2009. State and federal money reimbursed the city for its G20-related expenses, including the condos, he said.

Lamb, the controller, said he wants to see how police handle payments they receive for off-duty officers hired to work private security details. The credit union is not an authorized depository for city money, he noted. Lamb is also running for mayor against Ravenstahl.