BYRON YORK: When did Trump-Russia probe begin? Investigators focus on mystery months.

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.

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?

What, indeed?