This post has been updated with news Johnson will share what he knows with House impeachment investigators.

What did Sen. Ron Johnson know about President Trump’s intentions with Ukraine? That’s been a question from the start of this impeachment inquiry, ever since the senator himself acknowledged he heard from people there was a quid pro quo.

The Wisconsin Republican’s name keeps popping up in testimony in critical conversations, including with Trump and Ukraine’s president. They are conversations that could help assess whether Ukrainians knew there were conditions on getting their military aid and whether Trump was behind ordering those conditions.

New closed-door testimony Friday folds Johnson in again, this time detailing a September meeting in Ukraine with Ukraine’s newly elected president, Volodymyr Zelensky. Here’s U.S. diplomat in Ukraine David Holmes saying this in his opening statement, obtained by CNN, about Johnson:

President Zelensky asked about the security assistance. Although both senators stressed bipartisan congressional support for Ukraine, Senator Johnson cautioned President Zelensky that President Trump has a negative view of Ukraine and that President Zelensky would have a difficult time overcoming it. Senator Johnson further explained that he was “shocked” by President Trump’s negative reaction during an Oval Office meeting on May 23, when he and the Three Amigos proposed that President Trump meet President Zelensky and show support for Ukraine.

Why that’s notable: We know that Johnson wanted the security aid, which was mysteriously frozen after Congress approved it this summer, given to Ukraine. He’s the vice chair of the Senate Ukraine Caucus and a key member on the Senate Foreign Relations Committee who helped shepherd it through Congress.

But Johnson has been publicly very supportive of Trump, giving the president the benefit of the doubt that he was interested in ensuring the money wouldn’t be used corruptly. Yet here’s Johnson in a closed-door setting appearing to say he doesn’t understand why Trump isn’t more supportive of Ukraine.

Johnson has surfaced so much in key conversations that House Republican investigators want to talk to him. They likely see a way for Johnson to tell a potentially different, more Trump-friendly version of events than State Department and national security officials have so far. They asked him over the weekend to share what he knows, and Johnson said Sunday he will: " I will lay out what I know in terms of this, and to a certain extent some of my perspective," he said on NBC’s “Meet the Press.”

Where else has Johnson surfaced? Let’s revisit, starting by working backward.

In August, Johnson was actively trying to learn why the Trump White House froze the military aid to Ukraine. He talked to diplomat Gordon Sondland about it. Sondland, the U.S. ambassador to the European Union, told him Trump would release the aid if Zelensky announced an investigation to “get to the bottom of what happened in 2016.” Last month, Johnson detailed that conversation with Sondland and his reaction with the Wall Street Journal:

My reaction was: Oh, God. I don’t want to see those two things combined. Johnson said Trump adamantly denied a quid pro quo the next day, telling Johnson: “No way. I would never do that. Who told you that?”

The next day, Johnson actually called Trump about it. Here’s what Johnson told the Milwaukee Journal-Sentinel about that conversation:

“I didn’t succeed. But the president was very consistent on why he was considering it. Again, it was corruption, overall, generalized — but yeah, no doubt about it, what happened in 2016 — what happened in 2016, as relates? What was the truth about that? And then the fact that our NATO partners don’t step up to the plate.”

Why that’s notable: As The Fix’s Aaron Blake writes, Johnson quietly confirmed he heard from Trump there was a quid pro quo for the aid: “what happened in 2016.” And that he didn’t like that. His conversation with Trump would help illuminate whether Trump was behind the quid pro quo.

In July, the senator met with Andrii Telizhenko, a former Ukrainian diplomat who was known for circulating conspiracy theories about the Ukrainian government and the Democratic National Committee collaborating during the 2016 election, as reported last month by The Post’s Elise Viebeck, who was the first reporter to look at all of Johnson’s ties to this probe.

Why that’s notable: Trump’s personal lawyer Rudolph W. Giuliani was peddling this theory and meeting with this same Ukrainian diplomat. The conspiracy theory, batted down by former members of Trump’s own administration, made it all the way to Trump and he communicated it to Ukraine’s president in a phone call that same month.

In May, the senator attended the inauguration of Zelensky. That’s not an uncommon thing for a senator with interest in Ukraine policy to do. But what was interesting was whom Johnson was with: The “three amigos” referenced earlier — Sondland, diplomat Kurt Volker and Energy Secretary Rick Perry. (They gave themselves that name.)

Why that’s notable: This was the same time that Trump ousted the U.S. ambassador to Ukraine Marie Yovanovitch and instead sent his political appointees to Zelensky’s inauguration and had them oversee an informal diplomatic policy channel on Ukraine, frustrating the rest of the State Department.

Later that May, the “three amigos” and Johnson reported back to Trump in the Oval Office. Here Sondland testified that Trump told them to talk to Giuliani — which troubled him and would help connect the dots between frozen military aid and Giuliani’s political conspiracy theories.

Why that’s notable: In an interview with The Post, Johnson said he didn’t recall Giuliani’s name being mentioned. But either way, that Oval Office meeting in May with key players in Trump’s orbit on Ukraine seems to be a brainstorming session, and Johnson was in it.