In an ordinary case, that would not be a point worth making. The FBI routinely conducts major investigations in collaboration with Justice Department prosecutors — usually from the U.S. attorney’s office in the district where potential crimes occurred. That is because the FBI needs the assistance of a grand jury. The FBI does not have authority even to issue subpoenas, let alone to charge someone with a crime. Only federal prosecutors may issue subpoenas, on the lawful authority of the grand jury. Only prosecutors are empowered to present evidence or propose charges to the grand jury. And the Constitution vests only the grand jury with authority to indict — the formal accusation of a crime. In our system, the FBI can do none of these things.

No Justice Department, no grand jury. No grand jury, no case — period. As a technical matter, no matter how extensively the FBI pokes around on its own, no one can be a subject of a real investigation — i.e., one that can lead to criminal charges — unless and until there is a grand jury. That does not happen until the Justice Department hops on board.

Alas, regular criminal-justice procedures have been suspended by the explosive politics of the Clinton investigation. The FBI is doing its professional, apolitical best to investigate the presumptive Democratic nominee for president of the United States. The high stakes rattle not only the Clinton campaign but also the Democratic administration in which Mrs. Clinton worked when she engaged in the work-related conduct being investigated. On the one hand, the Obama administration does not want to be seen by the public as obstructing the FBI; on the other hand, President Obama does not want to be seen by his base as tanking the Democrats’ best shot at retaining the White House — the likely fallout if the Obama Justice Department signals that a formal, very serious criminal investigation is underway.

So Obama is hedging his bets. He is letting the FBI investigate, but on its own, without Justice Department prosecutors and the grand jury.