To use the popular cable-news vernacular, Mueller did not establish “collusion.” (Never mind that “collusion” is not a legal term, and that the special counsel’s mission was to investigate “links and/or coordination” with the Russians.)

Trump’s triumphant supporters notwithstanding, we don’t yet know what that means. When a prosecutor says that an investigation “did not establish” something, that doesn’t mean they concluded it didn’t happen, or even that they don’t believe it happened. It means the investigation didn’t produce enough information to provethat it happened. Without seeing Mueller’s full report, we don’t know whether this was a firm conclusion about lack of coordination or a frank admission of insufficient evidence. The difference is meaningful, both as a matter of history and because it may determine how much further Democrats in Congress are willing to push committee investigations of the matter…

Crucially, we don’t know whether Barr concluded that the president didn’t obstruct justice or whether he couldn’t obstruct justice. Well before his appointment, Barr wrote an unsolicited memo to Rosenstein arguing that Mueller’s investigation was “fatally misconceived” to the extent it was premised on Trump firing former FBI Director James Comey or trying to convince Comey to drop the investigation of Michael Flynn, Trump’s first national-security adviser. Barr’s memo was a forceful exposition of the legal argument that the president cannot obstruct justice be exercising certain core powers like hiring or firing staff or directing the course of executive branch investigations. So although Barr’s letter to Congress says that he and Rosenstein found no actions that constituted “obstructive conduct” undertaken with the requisite corrupt intent, we don’t know whether he means that Trump didn’t try to interfere with an investigation, or that even if he did it wasn’t obstruction for a president to do so. Democrats in Congress will want to probe that distinction—as they should.