Revelations that an FBI informant insinuated himself into the Trump campaign have led some congressional investigators to rethink their theories on how and why former President Barack Obama's Justice Department began investigating the 2016 Trump presidential effort.

Most reporting has focused on the July 31, 2016, creation of a document formally marking the beginning of the FBI counterintelligence probe targeting the Trump campaign. The document, known as the electronic communication, or EC, is said to have focused on the case of George Papadopoulos, the peripheral Trump adviser who has pleaded guilty to lying to special counsel Robert Mueller about his contacts with people connected to Russia.

[Click here for complete Robert Mueller coverage]

Most of the key events of the Trump-Russia investigation — the Carter Page wiretap, the wiretap of Michael Flynn's conversations, the presentation of Trump dossier allegations to the president-elect — took place after the formal start of the FBI counterintelligence investigation.

But now comes word of the FBI informant, described in various accounts as a retired American professor living in England. The Washington Post reported that, "The professor's interactions with Trump advisers began a few weeks before the opening of the investigation, when Page met the professor at the British symposium."

A few weeks before the opening of the investigation — those are the words that have raised eyebrows among Hill investigators. If it was before the investigation, then what was an FBI informant doing gathering undercover information when there was not yet an investigation?

The question has pointed investigators back to the issue of when the probe began — not when a piece of paper was formally signed but when the FBI, and perhaps other U.S. intelligence agencies, began investigating the Trump campaign.

And that has taken them back to March 21, 2016, when candidate Donald Trump met with the editorial board of the Washington Post.

At the time of that meeting, Trump had been under criticism for not having the sort of lists of distinguished advisers that most top-level campaigns routinely assemble. That was particularly true in the area of foreign policy. A frustrated Trump ordered his team to compile a list of foreign-affairs advisers.

Trump was preparing to announce his advisory board when he met with the Post. The paper's publisher asked Trump if he would reveal the names of his new team.

"Well, I hadn't thought of doing it, but if you want I can give you some of the names," Trump said. He then read a brief list, among them Page and Papadopoulos.

Trump's announcement did not go unnoticed at the FBI and Justice Department. The bureau knew Page from a previous episode in which Russian agents had tried, unsuccessfully, to recruit him. It's not clear what the FBI knew about the others. But then-Director James Comey and number-two Andrew McCabe personally briefed Attorney General Loretta Lynch on the list of newly-named Trump foreign policy advisers, including Page, according to U.S. officials familiar with the matter.

Lynch told the House Intelligence Committee that she, Comey, and McCabe discussed whether to provide a "defensive briefing" to the Trump campaign. That would entail having an FBI official meet with a senior campaign official "to alert them to the fact that … there may be efforts to compromise someone with their campaign," Lynch said.

It didn't happen, even though it was discussed again when Comey briefed the National Security Council principals committee about Page in the "late spring" of 2016, according to Lynch's testimony. (The principals committee includes some of the highest-ranking officials in the government, including the secretaries of State, Treasury, Defense, and Homeland Security, the attorney general, the head of the CIA, the White House chief of staff, U.N. ambassador, and more.)

So the nation's top political appointees, law enforcement, and intelligence agencies were watching Trump campaign figures in the spring and early summer of 2016.

In early July, Trump dossier author Christopher Steele, the former British spy, approached the FBI with the first installment of the dossier. (It was the part that alleged Trump took part in a kinky sex scene with prostitutes in a Moscow hotel in 2013.) Also in early July — just a few days later — Page made a much-watched trip to deliver a speech in Moscow. Also in July, FBI officials say they learned about Papadopoulos' meeting a few months earlier with a Russian-connected professor. And still in July, hacked emails from the Democratic National Committee were released.

Somewhere around the time all that was happening, according to the latest reporting, the FBI informant began his work.

And that was all before what is called the formal beginning of the Trump-Russia investigation. It is in those mystery months — late March, April, May, June, and early July of 2016 — with the presidential campaign going at full force, that the Obama administration's surveillance of the Republican candidate geared up.