The President's former national security advisor has been most helpful to Mueller in building a criminal case.
Michael Flynn, the President's former National Security Advisor, really, really cooperated with Robert Mueller's investigators. He sat for 19 interviews and, because of his cooperation and military service, Mueller says he should not go to jail or, potentially, even pay any fines. In fact, some of what Flynn provided is still being investigated by Mueller and it appears part of it could be directly related to a criminal investigation of President Trump.
According to court documents filed by the Mueller investigators, Flynn admitted to lying about his conversations with Russia's ambassador prior to the inauguration and also lying to the FBI. Flynn also lied to the Vice President.
Mueller's team claims that Flynn's cooperation also "likely affected the decisions of related firsthand witnesses to be forthcoming."
What is new in the document is Mueller revealing there is a previously unknown criminal investigation that may or may not implicate the President directly. Flynn also testified about the transition team's conversations with the Russians in the run up to the inauguration, though that seems likely not to be criminal.
We do know that Mueller is investigating whether the President tried to obstruct justice related to an investigation into Mike Flynn.
I do have to say that I think any conversations the President's transition team had with Russia are legal and legitimate as part of the transition, but I've also long suspected the President's team would get hung up on the President's private dealing with Russia. If the President really has a weakness in all this, I do not think it is collusion to steal an election, but using the Trump Organization as a pass through entity for Russian money.
Time will tell on all fronts.