In the final weeks of Philadelphia's heated 2003 mayoral campaign, FBI (search) agents crept into City Hall and installed a microphone above the mayor's desk.

For months, they had been gathering evidence that city contracts were being traded for campaign contributions. They wanted to know whether Mayor John F. Street (search) himself was involved.

Sixteen months later -- and weeks into the third federal trial to come out of the graft (search) investigation -- they are probably still wondering.

The bug in the mayor's office was in place for only a shotime before it was discovered by Philadelphia police and the investigation was cut short. During that time, the listening device picked up nothing of importance, FBI agents said.

FBI agents also secretly recorded more than 25,000 phone calls involving one of Street's top fund-raisers for nearly nine months. But none of the transcripts released in recent weeks have disproved Street's declaration that anyone eavesdropping on his conversations would find "no corruption, no sex and no profanity."

The FBI apparently overheard no instance in which the mayor said outright that he was willing to trade city work for a donation, and Street has not been charged with any wrongdoing.

The absence of a smoking gun has allowed his supporters to suggest the mayor's problems are probably behind him.

"I think a lot of people see that there are some serious questions that have been raised by the investigation, but there are also legitimate answers," said Street's spokesman, Dan Fee.

The tapes are being played in court this month for the trial of former city Treasurer Corey Kemp (search), who is charged with accepting tens of thousands of dollars worth of illegal gifts from one of Street's top campaign fund-raisers, a lawyer named Ronald A. White (search). Prosecutors said White -- who died in November while awaiting trial -- was trying to buy himself influence over the awarding of city contracts.

The recordings do not paint a flattering picture of city government. In several calls, the lawyer and treasurer can be heard talking about rewarding companies that had given money to Street's campaign, and denying city work to people who hadn't.

But Street's knowledge of the dealings between his treasurer and fund-raiser remains unclear.

In one typical call intercepted by the FBI, Street listened politely to White's request that a certain financial services firm be included in a city transaction, but stopped short of promising to take any action.

In another, Street listened as White described a plan to "sell" tickets in the city's luxury box at Philadelphia Eagles (search) games to campaign contributors. But the mayor seemed to throw cold water on the plan by saying seating in the box was limited.

Investigators also went looking for evidence that Street had used his office for personal gain, but may have come up short there, too.

Financial records seized by the FBI showed that Street and his son received loans from Commerce Bank (search) at a time when the bank was seeking city deposits. But there has been no proof that the transactions were linked to any city deal or conveyed the mayor any special benefit.

In another recorded call, obtained by the Philadelphia Daily News, White seemed surprised that investigators had suspected Street of wrongdoing.

"I just can't imagine them doing that to John, man," White told Kemp on the day the bug was discovered. "John is one of the most honest guys ever. I mean, this guy's squeaky-clean, man."

"Yes he is," Kemp said.

"Here's a guy living in North Philly. Ain't got no money," White said. "I just can't imagine what ... they're doing."

If the investigation has embarrassed or worried Street, he hasn't let on.

Street's aides say it has not hurt his ability to govern. Nor have Philadelphians seemed particularly outraged. There are occasionally boos for Street when he appears before certain audiences, but that is not unusual for Philadelphia, where people routinely razz public figures for sport.

City Councilman James Kenney accused Street last week of presiding over "one of the most corrupt administrations that I can remember." And other council members and at least one civic group have called for a ban on campaign contributions from people vying for no-bid city contracts.

"I think we need sweeping changes in campaign finance and the way contracts are awarded," said Zachary Stalberg, president of the watchdog group Committee of Seventy (search). "In Philadelphia and other cities experiencing the pay-to-play problem, we are going to need a whole set of reforms."

How the issue will affect the 61-year-old Street's political career is unclear. The mayor is limited to two terms, so he cannot run for re-election. Supporters have suggested he may be considering a run for Congress, but Street has said he is still years away from thinking about his post-mayoralty.

Through it all, Street has refused to answer questions about the probe, although that could change. Kemp's attorneys have said they intend to call the mayor as a defense witness.

"If anyone wishes me to testify, I will be there," Street said last month. "I will have my say and I will tell the truth."