This has become known because of the questions Mueller's team has been asking other witnesses.
If I hack Senator Noodle's email, that's an illegal act. If I quietly slip those stolen emails to the media and XYZ network breaks the story about that, the TV network has not committed an illegal act because it had no part in the hacking.
But what if I secretly speak with Noodle's opponent in this fall's election and say I hacked the senator's emails and am planning to release them? And what if Noodle's opponent says, "if you wait to release them after October 15, that would be perfect"? Has Senator Noodle's opponent committed a crime?
This is essentially what Mueller is trying to determine about Trump -- what did he know, when did he know it, and what did he do about it?
Later yesterday afternoon, following up on the NBC story, the Washington Post's Philip Bump established with an intricate timeline analysis (link here) which suggests Mueller thinks Trump knew about the Russia hacks in advance. If this is the case, then Mueller almost certainly is trying to determine if Trump broke the law.
If so, what that, in turn, leads to is yet to be determined. Some legal experts and scholars Mueller might seek to indict Trump himself, which would be the first time in history that a sitting President was charged with a crime. And given Trump is again warring with the Attorney General, who may have to sign off on any decision to indict the President, Donny may have nicely boxed himself into a corner.
Definitely stay tuned.