San Francisco District Attorney George Gascón announced Tuesday his office is launching a joint task force with the FBI to investigate political corruption in the city.

Gascón’s announcement comes a month after the district attorney and federal law enforcement officials disclosed criminal charges against three San Francisco political insiders with ties to Mayor Ed Lee.

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Stanford Law Professor Robert Weisberg said local political graft is often easier to charge in state court, because federal charges would have to involve crimes like wire fraud and other illegal activity that crosses state lines.

“You may well view this as a kind of cooperative loan of FBI resources to the DA,” he said, adding that it’s far from unprecedented for federal agents to help local prosecutors with investigations.