Since President Trump fired FBI Director James Comey on May 9, the explanations for the dismissal have been getting murkier. (Jenny Starrs/The Washington Post)

Big Russia investigation news: Special counsel Robert S. Mueller III has obtained an early draft of a letter President Trump wrote about his reasons for firing FBI Director James B. Comey. The news was first reported by the New York Times.

But, you might wonder, isn't it already pretty clear why Trump fired Comey? He went on NBC News and blurted out it was because of the Russia investigation! He said the Justice Department's recommendations were a ruse!

But things aren't quite that simple. That's why the letter could be so significant.

While it has often been reported Trump admitted he fired Comey because of the Russia probe — contrary to days of varied White House explanations — I've never really thought that was entirely accurate. At least not legally speaking. And likewise, Trump didn't so clearly admit he enlisted the Justice Department to put up a smokescreen.

Here's the exchange between Trump and NBC's Lester Holt (emphasis added):

HOLT: So, you had already made the decision?

TRUMP: Oh, I was going to fire regardless of recommendation.

HOLT: So, there was ...

(CROSSTALK)

TRUMP: They — made a recommendation. He's highly respected. Very good guy, very smart guy. And the Democrats like him. The Republicans like him. He had made a recommendation. But regardless of recommendation, I was going to fire Comey, knowing there was no good time to do it.

And in fact, when I decided to just do it, I said to myself — I said, you know, this Russia thing with Trump and Russia is a made-up story. It's an excuse by the Democrats for having lost an election that they should've won. And the reason they should've won it is the electoral college is almost impossible for a Republican to win. It's very hard because you start off at such a disadvantage. So everybody was thinking they should've won the election. This was an excuse for having lost an election.

HOLT: But were — are you angry … with Mr. Comey because of his Russia investigation?

TRUMP: I just want somebody that's competent. I am a big fan of the FBI. I love the FBI.

This may have been the most shocking moment in the entire Russia scandal. Comey's firing was instantly controversial given the FBI was investigating ties between the Trump campaign and Russia. But the administration had claimed the firing originated in the Justice Department and it was about Comey's conduct during the Hillary Clinton investigation. There were, of course, plenty of suggestions Trump really fired Comey because of Russia, but the administration made pains to say that wasn't it at all.

Then Trump spoke. In one fell swoop, the president admitted the DOJ's recommendation he fire Comey didn't matter, and he completely undercut the stated reasons.

What he didn't do, though, is explicitly admit he fired Comey because of the Russia investigation. He said it was on his mind at the time he made the decision, but he didn't connect the dots and say, “I did this because of the Russia probe.” As I've argued before, he left himself at least a hint of plausible deniability in the way he phrased it. And when Holt followed up and tried to connect those dots and get Trump to say it more explicitly, Trump deflected.

He also didn't say, in so many words, the Justice Department's recommendations were intended to deceive or offer a false justification for Comey's firing. That's where he could get into legal trouble for obstruction of justice. He said he was going to fire Comey regardless, but the White House has said he was merely checking all the boxes and getting as much input as possible. (This doesn't change the fact, of course, the White House said falsely that the decision originated in the Justice Department. This letter further exposes that massive falsehood.)

Which brings us to that letter. According to The Post reporters, who were briefed on its contents, it doesn't dwell on Russia, but it does express concern Comey wouldn't say publicly Trump himself wasn't under investigation.

The letter was drafted with top White House adviser Stephen Miller, a former Senate aide to now-Attorney General Jeff Sessions. Miller has been among the most controversial and ideologically extreme members of the White House staff, and it's tempting to believe he wouldn't rein in Trump's impulses in the drafting of the letter.

We also know the letter was multiple pages — suggesting plenty to pick apart — and aides cautioned Trump against at least some of its contents. The Times reports White House counsel Don McGahn believed the letter was “problematic.”

But just how problematic? That's the big question. Given Trump blew up the rationale for firing Comey mere days later — and he was involved in composing the letter himself — it's not unreasonable to think he might have shed a bit too much light on certain subjects.