PITTSBURGH -- The public couldn't avoid Luke Ravenstahl when he became the youngest mayor of a major U.S. city nearly seven years ago, even appearing on "Late Night with David Letterman."

Now a 33-year-old lame duck, Ravenstahl governs bunker-style and is rarely seen since a federal grand jury probe claimed his hand-picked police chief and now appears to be focusing on Ravenstahl himself.

For weeks, mayoral staffers – including three police bodyguards and a personal secretary – have been summoned to the seventh-floor grand jury room in the federal courthouse just a few blocks down brick-paved Grant Street from Ravenstahl's office. Witnesses or their attorneys have confirmed testimony about two main subjects:

_ Whether Ravenstahl used or sanctioned a slush fund that led to the police chief's resignation and indictment.

_ Whether the mayor instructed bodyguards to delete the actual times of day they worked from time cards, presumably to keep reporters from documenting the single mayor's nightlife that, otherwise, is chronicled only by rumors or cellphone pictures posted on gossipy blogs.

"The question becomes at what point does doing something like that – even if it would be inappropriate, even if it would be outside the parameters of the mayor's authorized security details – at what point does it become criminal?" said Bruce Antkowiak, a former federal prosecutor in Pittsburgh, now a law professor at nearby St. Vincent College.

Ravenstahl was 26 when, as city council president, he automatically succeeded Mayor Bob O'Connor, who died of brain cancer in September 2006. Ravenstahl enjoyed a honeymoon as Pittsburgh's youngest mayor, with even detractors hoping the cash-strapped city would reap the benefits of youthful energy rather than the ravages of immaturity.

Ravenstahl has balanced six straight budgets and raised the city's bond rating from junk status to its highest level in a decade. He also wooed the 2009 Group of 20 economic summit to Pittsburgh and benefited from other resurgences, such as a resurrected marathon.

But his tenure has also been marked by peevish behavior and scandal.

Ravenstahl divorced his high school sweetheart in 2011, explaining she was uncomfortable in the political limelight. Then, as the grand jury gathered steam this year, Ravenstahl first defended Nate Harper, the police chief, then demanded his resignation. In the same few weeks, Ravenstahl also announced he was running for another four-year term before abruptly quitting the race March 1.

Ravenstahl sounded more relieved than forlorn when he cited the "grueling demands" and collateral family damage as reasons for leaving office at year's end.

Since then, Ravenstahl rarely makes public appearances. When he has shown up, Ravenstahl has generally cut and run if reporters' questions veer to the investigation.

In general, the mayor has denied wrongdoing and questioned the motives of at least one former bodyguard, retired detective Fred Crawford, who testified before the grand jury in mid-June.

Defense attorney Robert Steward said Crawford testified mostly about the time-card issue and predicted the investigation will show taxpayers' dollars were "wasted so the mayor could have a designated driver ... while he went out to bars."

Ravenstahl's attorney, Charles Porter Jr., contends even if that happened and the guards were told not to specify what hours they worked – only how many – it's not a crime.

"So is it OK for the president to golf with Tiger Woods – and I'm guessing they had a beverage afterward?" Porter said. "How is that different than the mayor going out, even if he chooses to go out at a later time of the day and the taxpayers are paying for it, because we've decided it's OK to safeguard our officials when they're in public?"

Porter also dismissed questions about Ravenstahl's connection to the slush fund that ended Harper's career, saying "I don't have any concern there."

Harper's attorneys have said he will plead guilty this year to wrongly diverting fees that bars and nightclubs pay when hiring city officers for off-duty security work into a slush fund used to make some police-related purchases off the city's books. Harper is also charged with spending more than $30,000 from the fund on personal matters.

"I'm hard-pressed to understand how any of this is worthy of a federal criminal prosecution" of Ravenstahl, Porter said.

U.S. Attorney David Hickton won't comment or even confirm the grand jury investigation, let alone say whether Ravenstahl is a "target" – someone prosecutors expect to arrest – or merely a "subject" – someone whose actions are being questioned and who may or may not become a target.

A recent U.S. Supreme Court case could make it even harder to fasten a bull's-eye on Ravenstahl.

In a 2010 case involving former Enron CEO Jeffrey Skilling, the high court ruled that "honest services fraud" – a crime experts say seems to be in play with the grand jury – could be prosecuted only in cases involving bribes or kickbacks.

In other words, public officials like Ravenstahl can be prosecuted for peddling their influence, but not just for misusing taxpayer-funded resources like bodyguards.

"You have to make a distinction between things like the mayor having his office painted in an elaborate color of paint that would have cost more than the government-issue kind of paint," Antkowiak said, exaggerating to make the point.

"You may take him to task for that in the press, but you don't take it before the grand jury."

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