Who Authorized FBI Surveillance of Gen. Petraeus?

Even by the standards of the friday afternoon news dump tradition, yesterday's news that Gen. David Petraeus had resigned from the CIA was a stunner. The most heralded military officer of his generation fell from grace after it was revealed he'd had an affair with his biographer. The affair was discovered earlier this year soon after the FBI began monitoring his email. This raises a very serious question. Who authorized the FBI to monitor the e-mail of the Director of the CIA?

 
The Director of the CIA is one of the central individuals responsible for America's national security and intelligence. His communications would, by their very nature, be one of the nation's closest guarded secrets. According to FBI sources, the Bureau began surveillance of his email communication in the Spring of this year, after an email Petraeus sent to his biographer, Paula Broadwell, was misinterpreted to reference possible corruption. 

First question: How did that initial email come to light?

Second question: Who authorized the Bureau to pore over all his email communication? We are talking, after all, about the Director of the CIA. That cannot have been a decision made by a junior staffer at DOJ. I doubt that even the Director of the FBI could authorize the surveillance of a critical member of America's national security team.
Is this a decision Eric Holder can make? Or, does it come under the exclusive purview of the White House?

Third question: Who were the agents given access to Petraeus' communications? Presumably, they read a lot of emails that weren't related to the past affair and likely contained top secret and classified information. What security clearance did these agents have?

According to several news accounts, the FBI investigation uncovered thousands of emails between Petraeus and Mrs. Broadwell. The nature of their relationship would have been clear immediately.

So, exit question. If the investigation began in the Spring, why did the Bureau sit for so long on the revelations about the affair? We are told now that an extramarital affair by the Director of the CIA is itself a breach of national security. If that's true, why didn't the FBI move immediately to force his resignation?
Of course, in an election year, this is a question which answers itself.