THE DIRECTOR of the FBI, James B. Comey, did the right thing in announcing the results of the bureau’s investigation of Hillary Clinton’s email in early July. Realizing that the case was hyper-sensitive in the middle of a presidential campaign, Mr. Comey spoke up when ordinarily he would have simply forwarded his recommendation to prosecutors. He said the investigation determined that she was “extremely careless” in using a home-brew server while secretary of state, but that Ms. Clinton’s actions did not warrant prosecution. It was important for the apolitical FBI director to say one way or the other whether there was criminal behavior so voters could make up their own minds.

But now Mr. Comey has taken a misstep. The FBI has provided to Congress portions of the investigative files from the year-long probe. Although it is not known precisely which portions, some of the materials include interviews conducted by the FBI. On Capitol Hill, the FBI has deposited the material in a Sensitive Compartmented Information Facility, or SCIF, a room with restricted access.

According to the Associated Press, the documents were transmitted to Congress with written warnings not to leak them. “These materials are nonpublic and contain classified and other sensitive material,” FBI Acting Assistant Director Jason Herring wrote. “For that reason, these materials may not be further disseminated or disclosed, in part or in full, without obtaining the FBI’s concurrence.” Access to the documents is restricted to members of the judiciary, intelligence and government affairs committees.

Republicans in Congress are unlikely to heed the warnings. Are these members of the GOP, who have enthusiastically exploited the Benghazi and email stories for partisan advantage, whose convention delegates chanted “lock her up,” really going to read the FBI files and stay mum? The temptation here for mischief — partial leaks — is enormous.

It is extremely rare for the FBI to turn over to Congress internal case files from a criminal investigation that did not lead to a prosecution. In some cases in the past — counterintelligence probes, for example — the FBI has orally briefed Congress or, in the case of nominations, brought the raw interview notes to senators to read but not keep.

In this case, it seems a bad precedent to put the Clinton investigative materials before Congress. Will lawmakers demand more such sensitive documents in the future? If informants know their cooperation with the FBI might eventually be shared with members of Congress, will it have a chilling effect?

At this point, the best course of action, and the most transparent, would be to put the documents through a suitable declassification process and then release them for all to see. But it shouldn’t have come to that.