Street's announcement came only one day after Police Commissioner Sylvester M. Johnson said he had seized Mitchell's gun permit, which was issued by someone in the department even though state police had twice rejected Mitchell's application for a permit.
"It's not a lawful permit," said Street, who was accompanied by Johnson at the news conference. "A fraudulent application was issued, and that's a crime. The deputy was the subject of the application. And we have to get to the bottom of this."
Mitchell, 68, was arrested in the 1950s on charges of impersonating a police officer. That arrest, which occurred when Mitchell was a 19-year-old civilian, was uncovered when state police did a routine background check in response to his request for a gun permit.
Under state law, Mitchell could have appealed the decision, but the deputy commissioner never did so, Street said. Instead, someone in the Philadelphia department issued the bogus permit.
The mayor said he was alarmed that a phony document - bearing a false identification number - had been created for Mitchell.
"There is an appeal process. The appeal process was not followed," Street said.
Police said Mitchell had applied for the gun permit because he had planned to retire last November and wanted to continue carrying a weapon.
Last summer, however, Street announced that he had persuaded Mitchell to postpone his retirement. At the time, he said he wanted Mitchell to stay on the job four more years.
Street vowed that a police Internal Affairs investigation would determine how that permit was issued.
Meanwhile, he said, Mitchell will be on administrative leave. He declined to speculate how long the investigation might take. He said that any evidence of criminal actions would be turned over to the District Attorney's Office for possible prosecution.
Under normal procedures, anyone applying for a gun permit in Philadelphia is asked a series of questions about past arrests. Even if a person has been acquitted of a crime, an applicant for a gun permit must give all details of past arrests. Any deliberate false statement to those questions is a criminal act, police say.
In the case from the 1950s, Johnson has said Mitchell was acquitted well before he joined the Police Department in 1958.
Street said investigators are focused on how Philadelphia police issued Mitchell's permit last year - not on whether the state police had been correct in denying his application and not on the arrest from the 1950s.
The Police Department also issued Mitchell a gun permit in 1995, although state police had denied Mitchell a gun permit that year as well. But any crimes that might have been committed with the 1995 permit probably would be outside the statute of limitations for prosecution.
While Mitchell is suspended, he will continue to receive his $112,354 salary, Street said. Because he is still a member of the department, he will keep his gun and his badge.
Although Johnson accompanied Street in the news conference, he did not make a single comment. In Johnson's news conference Thursday, the commissioner had said he had complete confidence in Mitchell.
One reporter asked Street whether his announcement yesterday reflected a lack of confidence in Johnson.
"When there is an issue of integrity involving the No. 1 and No. 2 cops in the city . . . it may be appropriate for the mayor to be involved," Street said in response.
Contact staff writer Mark Fazlollah at 215-854-5831 or email@example.com.