When The Washington Post reported Wednesday that an intelligence community whistleblower was so alarmed about a "promise" President Trump made to a foreign leader that they filed a formal complaint, suspicion immediately fell on Russia or North Korea. But on Thursday, the Post and The New York Times reported that at least part of the allegations deal with Ukraine.

Here's one prominent theory:

The theory — that Trump was withholding $250 million in congressionally approved military aid to Ukraine until Ukraine's new president, Volodymyr Zelensky, produced dirt to harm former Vice President Joe Biden, the Democratic frontrunner to challenge Trump in next year's election — isn't new. It was the thrust of a Sept. 5 Washington Post editorial, for example, and Congress launched an investigation four days later. But Trump's direct involvement would be a big deal. Here's a timeline of how things seem to have played out:

May 10: Rudy Giuliani, Trump's lawyer, calls off a trip to Ukraine one day after saying he would be "meddling in an investigation" and giving Ukraine's new government "reasons why they shouldn't stop it because that information will be very, very helpful to my client." He lobbies Ukraine via back-channels over the summer.

July 25: Trump and Zelensky talk on the phone, and Trump, according to Ukraine's readout of the call, signaled Ukraine should "complete investigation of corruption cases, which inhibited the interaction between Ukraine and the USA."

Aug. 28: Politico reports that Trump has frozen $250 million in Ukraine security aid and Congress doesn't know why.

Sept. 1: Vice President Mike Pence meets with Zelensky in Poland. He's asked about Biden and the frozen funds the next day.

Sept. 9: The House Foreign Affairs, Intelligence, and Oversight committees announce an investigation into whether Trump and Giuliani "have increased pressure on the Ukrainian government and its justice system in service of President Trump's re-election campaign." They ask for a transcript of Trump's July 25 call with Zelensky and relevant documents.

Sept. 10: Intelligence Committee Chairman Adam Schiff (D-Calif.) writes Acting Director of National Intelligence Joseph Maguire to ask about the whistleblower complaint.

Sept. 11: The White House releases military aid to Ukraine with no explanation.

Sept. 19: Ukraine is reported to be a focus of the whistleblower's complaint. Giuliani goes on CNN and says he pressed Ukraine's government to investigate Biden. Then he suggests on Twitter that Trump should push Ukraine to investigate Biden.

As of Friday, the Trump administration was refusing to turn over the details of the whistleblower complaint, and Schiff was threatening to sue, telling reporters that "someone is trying to manipulate the system."

Stay tuned.

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