In an explosive new book, former White House strategist Steve Bannon describes as "treasonous" the June 2016 meeting between Donald Trump's campaign officials and a Russian lawyer to discuss an alleged offer to provide dirt on his opponent Hillary Clinton.

The meeting, which took place in Trump Tower in New York, is now one focal point in a sweeping investigation by three congressional committees and special counsel Robert Mueller into Russia's interference in the presidential election and possible collusion with Trump associates.

Not long after the meeting's existence was first reported in July of last year, Bannon told the author of Fire and Fury: Inside the Trump White House: “The three senior guys in the campaign thought it was a good idea to meet with a foreign government inside Trump Tower in the conference room on the 25th floor – with no lawyers. They didn't have any lawyers.

“Even if you thought that this was not treasonous, or unpatriotic, or bad shit, and I happen to think it's all of that, you should have called the FBI immediately," he continued.

The excerpts from Michael Wolff's book were published Wednesday by The Guardian, which obtained a copy. The book, reportedly based on more than 200 interviews, is set to be released next week by publisher Henry Holt and Co.

Bannon, a self-styled maverick who championed alt-right causes, was pushed out of the administration in August following a clash between white supremacists and counter-protesters in Charlottesville, Va. Bannon has since returned to Breitbart News and said he resigned of his own accord.

The Trump Tower meeting, which took place one month before Bannon joined the campaign, included President Trump's oldest son, Donald Trump Jr., along with his son-in-law and adviser Jared Kushner and then-campaign chairman Paul Manafort.

The campaign officials met with Russian lawyer Natalia Veselnitskaya and other Russians. An intermediary promised Trump Jr. information that would potentially damage Clinton's campaign. Instead of alerting the FBI, Trump Jr. replied in an email, "I love it."

Trump Jr. released the emails that led to the meeting last year, and has publicly given his rationale for taking the meeting. "To the extent they had information concerning the fitness, character or qualifications of a presidential candidate, I believed that I should at least hear them out," Trump Jr. said in September.

Veselnitskaya reportedly told Senate investigators that Trump Jr. lost interest in the meeting after she said she didn't have evidence of illegal donations to the Clinton Foundation. But she insisted that he said the U.S. would re-examine sanctions against Russia if his father won the election.

Bannon also said that he expects Mueller's investigation into alleged collusion between the Trump campaign and Russia to focus on money laundering. “They're going to crack Don Junior like an egg on national TV,” Bannon tells Wolff.

Per Wolff's publisher, the book will also dive into the firing of former FBI director James Comey, what Trump's staff "really thinks of him" and why Bannon and Kushner couldn't be in the same room.

Wolff is a journalist and columnist who has contributed to USA TODAY and other publications.

According to New York magazine, which also published an excerpt of his book, he was allowed significant access to the West Wing, via "something like a semi-permanent seat on a couch."