As longtime keeper of Trump's secrets, Michael Cohen could make 'dangerous' witness, experts say Michael Cohen handled both business and personal matters for Donald Trump.

As Donald Trump’s personal attorney and problem solver, Michael Cohen handled both business and personal matters that ranged from the mundane to the highly sensitive.

Now, sources tell ABC News Cohen is likely to cooperate with federal investigators, and his lawyers are expected to leave the case. The role Cohen could play in any investigation that explores President Trump’s conduct remains uncertain, but legal experts say he could emerge as a crucial source of insight to prosecutors.

“He could be extremely valuable,” said Matthew G. Olsen, a former federal prosecutor and ABC News contributor. “He was not just a personal lawyer but also was President Trump’s so-called fixer for a number of years. So he would have had access to lots of very personal information involving his business dealings.”

Those dealings cover the broad sweep of the Trump’s global empire – including several projects that have caught the attention of federal investigators. Cohen played an integral role in early discussions about a possible Trump Tower in Moscow – negotiations that were going on during the early months of the 2016 presidential campaign.

Cohen has confirmed he attended a lunch meeting with a Ukrainian politician one week after Trump took office, where the two men discussed the potential for Cohen to share a Ukraine peace proposal with his contacts at the White House.

And Cohen’s name appeared repeatedly in the now infamous dossier of unverified allegations, which included salacious claims about Trump, prepared by former British intelligence agent Christopher Steele. The agent, who was hired by an opposition research firm that was paid initially by Republicans and later by Democrats, alleged Cohen’s involvement in attempting to covering up contacts between Russian operatives and members of Trump campaign.

Cohen fiercely denied the claims. In January, he tweeted “Enough is enough of the #fake #RussianDossier” before filing a lawsuit against Buzzfeed, the news outlet that first published the document – a suit he later withdrew.

As Cohen faces the mounting pressures of a federal criminal investigation, and may be preparing to part ways with his initial defense attorney, there are a range of topics prosecutors may want to explore with him.

“This is a deep intricate relationship built up over many years,” said Seth Hettena, the author of Trump/Russia: A Definitive History. “I think he’s dangerous in a bunch of different ways.”

Most notably, Hettena said, Cohen could shed light on what, if anything, Trump knew about non-disclosure agreements reached with Stormy Daniels and Karen McDougal, two women who allege they had sexual encounters with Trump years before his presidential run.

Daniels, whose legal name is Stephanie Clifford, entered into a then-secret agreement with Cohen less than two weeks before the 2016 presidential election.

After the Wall Street Journal first reported in January on the hush-money deal with Daniels – which paid her $130,000 in exchange for her silence – Cohen claimed that he had paid her out of his own personal funds and that he had not been reimbursed by the Trump Organization or the campaign.

On April 5, four days before the authorities raided Cohen’s properties in New York, President Trump told reporters on Air Force One that he didn’t know why Cohen had paid Daniels or where he had gotten the money to pay her. The president later acknowledged, in a financial disclosure form filed last month with the Office of Government Ethics, that he had reimbursed Cohen.

Then there is McDougal, who in August 2016 signed a $150,000 deal with American Media Inc., the publisher of the National Enquirer, that transferred to the company the rights to her story of an alleged ten-month romantic affair with Trump. The magazine never published her story. McDougal alleged in a lawsuit filed earlier this year that Cohen had allegedly conspired with her former attorney to bury the story. McDougal settled her lawsuit.

President Trump, through his representatives, has denied the allegations of McDougal and Daniels.

But Cohen, Hettena said, is “the guy who knows where all the bodies are buried.”