Jeff Sessions made a false statement during the Senate Judiciary Committee’s confirmation hearing on his nomination to be Attorney General. Answering a question from Senator Al Franken, of Minnesota, about contacts between the Trump campaign and Russia, Sessions said, “I have been called a surrogate at a time or two in that campaign and I did not have communications with the Russians.” But as the Washington Post first reported last week, and as Sessions himself later acknowledged, he did have two meetings with the Russian Ambassador, Sergey Kislyak. President Donald Trump, rarely accused of having a gift for understatement, gave one when he said of his Attorney General, “He could have stated his response more accurately.”

In light of these revelations, Sessions has recused himself from any participation in Justice Department investigations of matters relating to Russia and the Trump campaign. These probes will now surely include an examination of whether Sessions’s own statements to Congress were violations of criminal law. False statements before Congress can lead to charges of perjury (felony lying under oath), making a false statement (also a felony but for statements that are not necessarily under oath), or for withholding information from Congress (a misdemeanor that has been a popular plea bargain for Washington figures such as the former Attorney General Richard Kleindienst and the former national-security adviser Robert McFarlane). Partisans on both sides have been quick to convict or exonerate Sessions of any wrongdoing, but no final judgment is appropriate at this time. What’s needed is a full investigation—and here’s a guide to how such an inquiry might proceed.

In public statements after the Post’s disclosure, Sessions and his supporters offered various defenses of his conduct. They said Sessions didn’t remember his meetings with the Russian Ambassador; they said Sessions was meeting with Kislyak in Sessions’s role as a senator on the Armed Services Committee, rather than as a Trump campaign surrogate, which, they argued, made his response to Franken technically accurate, given the question he’d been asked; they said, furthermore, that the conversation between Sessions and Kislyak involved diplomatic, not campaign, matters, an additional indication that Sessions’s statement to the committee was accurate in context. These defenses may have merit. All the criminal laws involving false statements to Congress are so-called specific-intent crimes; in order to be found guilty, the defendant must intend to deceive. A faulty memory is a good defense. Likewise, establishing that a statement is literally true, even if misleading, is also a valid defense.

But all of Sessions’s possible defenses, as well as the details of a possible case against him, rest on facts—facts that are not yet known. This is what an investigation should disclose. For example, was anyone else present during the meetings between Sessions and Kislyak? If so, that person might confirm whether campaign matters were discussed, and if so, how much of a focus of the conversation they were. A third party could also help determine how long the meetings were; it’s easier to forget a meeting that lasts ten minutes than one that goes on for an hour. Likewise, how often did Sessions meet with Kislyak, or with any ambassador? There is nothing inappropriate about a senator meeting with an ambassador, but if Sessions had such encounters only rarely, it would be harder to credit his explanation that he simply forgot this one.

As in most investigations, the key evidence is likely to be documents, rather than testimony, because contemporaneous records are often more reliable than memories. Did Sessions or an aide take notes about the Kislyak meeting? Are there e-mails setting it up? (If a Sessions office e-mail said, “The Ambassador wants to talk about arms control,” that would be significant and exculpatory; if an e-mail said, “The Ambassador wants to talk about the campaign,” that would be relevant in a different way.) Did Sessions himself get e-mails or memos from his staff about the Kislyak meetings in advance or as follow-ups? Those would provide useful clues about what was said between the two men.

Much of the evidence about the Sessions-Kislyak meetings would come from Senator Sessions’s official files—and this would raise a difficult issue for Justice Department investigators. Article I of the Constitution states, “for any Speech or Debate in either House, they”—that is, senators or representatives—“shall not be questioned in any other place.” The Speech or Debate clause, as it’s known, was designed to protect legislators from prosecution by the executive branch for espousing unpopular views. But, in a series of cases over the years, the Supreme Court has interpreted the clause in a way that gives legislators broad, but not unlimited, protection for their discussions in and for the operations of their offices. As a technical legal matter, Sessions could argue that any attempt by Justice Department investigators to interview his staff or examine the records of his senatorial office violates the Constitution.

Such an argument, however, would present grave political difficulties for the Attorney General. Since Sessions has recused himself from any investigation related to the campaign, his subordinates would be running the inquiry, presumably under the supervision of Rod Rosenstein, the veteran prosecutor who has been nominated to be Deputy Attorney General. Would Attorney General Sessions refuse to coöperate with an investigation his department was carrying out? Would his private lawyer—and he surely will get one, if he hasn’t already—litigate against the Justice Department in order to protect the records of his Senate office? It’s hard to imagine an Attorney General doing so—and remaining in office. But an investigation of the statements of a sitting senator in the course of a congressional hearing almost guarantees constitutional complications, if not outright conflict. At this point, of course, there is no way of knowing if Sessions will be charged, much less convicted, of any crime, but it is a certainty that an investigation of his activities will be a difficult and painful experience for him, for the Trump Administration, and, perhaps, for the country.