President Trump unleashed a brief burst of frustrated tweets Friday morning, focused on recent reports from The Post and others that the investigation into Russian meddling in the 2016 election had expanded into an investigation of whether or not he’d tried to interfere with that probe.

His tweets railed against the “fake news” media and how there was no proof that his campaign (or, in his phrasing, he himself) colluded with Russia to affect the outcome of the election. (He also incorrectly claimed that the investigation had only been going for seven months: It began last July.)

It was the fourth tweet in that series, though, that prompted the most questions.

For example:

1. Who, exactly, is Trump talking about here? We know that the “FBI director” is James B. Comey, whom he fired in early May. But who is “the man who told me to fire the FBI director”?

We know two things about that second person from Trump’s tweet. That person told him to fire the FBI director, and that person is investigating him.

At first pass, that would seem to indicate that he’s referring to Deputy Attorney General Rod J. Rosenstein, who appointed former FBI director Robert Mueller to serve as special counsel, leading the independent investigation into the Russia affair.

Rosenstein also wrote a letter last month outlining concerns about Comey that Attorney General Jeff Sessions then passed on to Trump with the recommendation that Comey be fired.

While that seems like it fits with Trump’s description, then — it actually doesn’t. First of all, Rosenstein’s letter never called for Comey’s firing. (It’s also worth noting that Trump told NBC’s Lester Holt that he planned to fire Comey anyway.) Second of all, the description of Rosenstein as investigating Trump is a bit off. The special counsel is investigating Trump, and Rosenstein can fire Mueller if he wishes, but he’s not in charge of that investigation. Rosenstein also has jurisdiction over the FBI’s investigation into the Russia matter.

So maybe Trump’s actually referring to Mueller? Mueller’s certainly investigating him — but there’s no indication that Mueller told Trump to fire Comey.

The safest answer: Trump is referring to Rosenstein — and trying to impugn the deputy attorney general by ensnaring him in the firing of Comey at the outset. Which raises another question …

2. Is Rosenstein’s role in the matter tainted? Our Matt Zapotosky raised this point on Twitter.

This issue of his letter to Trump about Comey was not a point of concern when Rosenstein first appointed Mueller. Of course, at that point the investigation wasn’t into Trump’s alleged attempt to lean on Comey to curtail the investigation into Michael Flynn. ABC News reported on Friday that Rosenstein had privately acknowledged to friends that he might need to recuse himself for that reason.

Management of the Justice Department is in a tricky position at the moment. Jeff Sessions, its head, recused himself from Russia-related matters after it was revealed that he hadn’t reported contacts with the Russian ambassador. That’s why it was Rosenstein who appointed Mueller. And as Rosenstein testified earlier this week on Capitol Hill, he’s currently therefore the one who can remove Mueller from the picture.

If pressure on Mueller continues to build, though, it’s easy to see how Trump might pressure Rosenstein to remove himself from that decision-making position. Were Rosenstein to concur and recuse himself from oversight of the special counsel, the duty would fall to Rachel Brand, associate attorney general.

After Brand, there aren’t many other appointees left to lead the Department of Justice, even when looping in the three U.S. attorneys who were added to the chain of succession by Trump earlier this year. If Trump wants to put pressure on people to fire Mueller, he’s running low on options.

3. Is this related to Rosenstein’s odd statement on Thursday night? Late Thursday, Rosenstein issued an unusual statement.

It read:

Americans should exercise caution before accepting as true any stories attributed to anonymous ‘officials,’ particularly when they do not identify the country — let alone the branch of agency of government — with which the alleged sources supposedly are affiliated. Americans should be skeptical about anonymous allegations. The Department of Justice has a long established policy to neither confirm nor deny such allegations.

Why this was released isn’t clear. We know from other reporting that Trump has, in the past, demanded that those working for him offer public defenses of him. One way to read this statement is that Rosenstein is trying to introduce cause for doubt that reports about an investigation to Trump have any basis in reality — an effort that seems like it would meet with Trump’s approval.

But, then, how do we reconcile that with Trump’s apparent disparagement of Rosenstein this morning? Was this statement something that Trump didn’t actually want to have released? Does it not say what Trump hoped? Or was Trump’s tweet completely independent of Rosenstein’s statement?

4. Why does he keep referring to this as a “witch hunt”? The point of Trump’s tweet, as is often the case, is to offer a defense of bad news to his base of support. (One of his tweets on Friday morning celebrated that his social media accounts let him speak directly to his supporters in that way.) Trump has come to use “witch hunt” the way he used “crooked Hillary” and “lyin’ Ted” on the campaign trail, and in the way he uses “fake news” in his tweets since becoming president.

A “witch hunt” is a baseless campaign to smear an innocent person, often one who holds views outside the mainstream. That’s not what the special counsel’s investigation is.

5. Speaking of which: Is Trump actually under investigation? It is amazing to say, but if we had only Trump’s tweet to guide our answer to this, it would still not be totally clear.

Trump has a distinct habit of making sweeping assertions on Twitter that he (or, often, his staff) then have to reel back in. (See the first question above.) Were there no other evidence at hand, we would be justified in being wary about Trump’s assertion about the investigation. Or put another way: On Twitter, we can’t always take Trump at his word. (Update: Sure enough, a Trump source denied that the president was confirming an investigation.)

In this case, though, Trump’s assertion that he’s being investigated is backed up by outside reporting. So, yes, Trump is under investigation, by Robert Mueller — and he’s annoyed enough about it that he’s taking aim at any- and everyone involved.