It's been another big week in the investigation into Russian meddling in the U.S. elections.

It culminated in a tweet Friday from President Donald Trump. He seems to be confirming he's under investigation for the firing of former FBI Director James Comey.

Here's the tweet: "I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt"

From the tweet, it sounds like Trump is blaming Deputy Attorney General, Rod Rosenstein.

Why would he do that?

Rosenstein serves under Attorney General Jeff Sessions. In March, Sessions recused himself from decision-making in the investigation. That left Rosenstein in charge of the Russia probe.

In early May, on the day Comey was fired, Rosenstein wrote a scathing memo about the way Comey conducted himself during the Hillary Clinton email investigation.

"As a result, the FBI is unlikely to regain public and congressional trust until it has a Director who understands the gravity of the mistakes," he wrote.

Intially, the Trump administration pointed to that memo to justify Comey's firing.

Was the Rosenstein memo the reason Comey was fired? Or did Trump fire Comey as a way to stop the Russia investigation from going forward? We don't know the answer to those questions yet.

We do know, it's Rosenstein who appointed Bob Mueller as Special Counsel.

And now, Trump's tweet seems to confirm Mueller is investigating the president. That's an expansion of the investigation beyond a focus on Russian influence.

Now, it's Mueller's job to find out if how all this really went down and if the president did anything wrong.