After refusing to testify in public last month, Glenn Simpson, a key figure behind the infamous Russia dossier with allegations of collusion by President Donald Trump's top aides, finally spoke with staff on the Senate Judiciary Committee on Tuesday.

The meeting was held in a private setting and stretched for more than 10 hours, a report said citing sources.

Simpson was previously a reporter for the Wall Street Journal, after which he founded Fusion GPS, a private research company. This was the firm that hired a British intelligence officer, Christopher Steele, during the 2016 presidential campaign and who produced the controversial dossier.

The 35-page document was filled with unsubstantiated allegations against Trump, along with intelligence about alleged meetings between his campaign and Russian operatives. 

During the closed-door meeting, Simpson gave the Senate committee over 40,000 documents, a source told Fox News. "Mr. Simpson told Congress the truth and cleared the record on many matters of interest to congressional investigators," his attorney Josh Levy told the news outlet.

Simpson is the first of three key people in the ongoing Russia probes to speak with the judiciary staff.  The other two, Donald Trump Jr. and former Trump campaign chairman, Paul Manafort, have also made arrangement with the judiciary committee to be privately interviewed ahead of any public session, CNN reported.

Simpson was served with a subpoena July 21 ordering him to testify, however, his lawyer said he was on vacation and would be traveling outside till early August.

Simpson communicated through his lawyers his intention of not testifying under the Fifth Amendment.

Sen. Chuck Grassley, (R-Iowa), released a statement July 26 about withdrawing the subpoena to Simpson. "I withdrew the subpoena after Mr. Simpson agreed to a transcribed staff interview about (1) his involvement in creating the Trump dossier and interactions with the FBI and (2) his alleged work with the unregistered foreign agents who met with the Trump family and campaign officials."

Grassley and his committee are investigating into Simpson's alleged failure to register as a foreign lobbyist, an allegation that Simpson's team has denied, CNN reported.

"Let's be clear about what's really happening: the President's political allies are targeting Fusion GPS because the firm was reported to be the first to raise the alarm over (the) Trump campaign's links to Russia," reports said citing an unsigned statement by Fusion GPS, which was delivered to the committee last month.

The dossier has remained a "source of intrigue" to those questioning the president's alleged links with Moscow. The dossier has also been a sore point for Trump, who has frequently vented out his ire over the issue, referring to those publishing the allegations in the document as "fake news." 

Days after BuzzFeed obtained the dossier and published it in January, Trump was outraged. “It’s all fake news. It’s phony stuff. It didn’t happen,” Trump said at a press conference. “And it was gotten by opponents of ours, as you know, because you reported it and so did many of the other people. It was a group of opponents that got together — sick people — and they put that crap together," the president was quoted by reports.