The Justice Department is widely reported to have offered criminal immunity to Bryan Pagliano, the techie who set up Hillary Clinton’s private email operation. Now that he has that get-out-of-jeopardy-free card, Mr. Pagliano also ought to be able to tell Congress and the public what he knows.

Last year Mr. Pagliano exercised his Fifth Amendment right not to testify about his role in maintaining a private email system for Mrs. Clinton when she was Secretary of State. But as GOP Senators Ron Johnson and Chuck Grassley wrote...