The longtime chief financial officer of the Trump Organization receiving immunity from federal prosecutors could be a "momentous turn of events" for President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE, according to biographer Tim O'Brien.

O'Brien, who penned the nonfiction "Trump Nation" in 2005, wrote in Bloomberg News on Friday that Allen Weisselberg's immunity in the investigation into former Trump attorney Michael Cohen could unearth a great deal about the president's past financial dealings.

ADVERTISEMENT

"Depending on how prosecutors proceed, it may take the federal tax- and bank-fraud investigation of Cohen — and, more important, Special Counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s probe involving Russian interference in the 2016 election — out of some of the relatively low-stakes legal issues in play so far and into the heart of the Trump Organization and the president’s business and financial dealings," wrote O'Brien, who now works as the executive editor of Bloomberg Opinion.

He adds that Weisselberg's cooperation with federal investigators will clearly agitate Trump due to the long history the CFO has working inside the Trump Organization. O'Brien writes that Weisselberg is "deeply familiar" with Trump's business and was relied on by Trump to sign off on the company's most important deals.

"Trump has had few loyalists in his operation historically, and Weisselberg is certainly one of the most longstanding members of that tiny club," O'Brien wrote. "His cooperation with investigators will resonate personally for the president, and is likely to force an already combative man to lash out in ever more forceful ways."

The comments from O'Brien came just hours after The Wall Street Journal first reported that Weisselberg was granted immunity.

The development occurred a few days after Cohen pleaded guilty to eight counts of bank fraud, tax fraud and campaign finance violations tied to the hush-money payments given to women who claim to have had affairs with Trump.

In addition to Weisselberg, prosecutors also granted immunity to National Enquirer publisher David Pecker this week as part of its probe of Cohen. Pecker has reportedly discussed with prosecutor's Cohen's involvement in the president's alleged nondisclosure payment deals with women prior to the 2016 presidential election.