Cohen has sent repeated signals that he is likely to cooperate in exchange for leniency from prosecutors.
Reminded that he had previously vowed to "take a bullet" or "do anything" to protect the president, Cohen told ABC in July that Trump is not his top priority.
"To be crystal clear, my wife, my daughter and my son, and this country have my first loyalty," he said.
Last month, Cohen lawyer Lanny Davis released an audio recording of a September 2016 conversation between Trump and Cohen in which they discussed a deal that a Playboy model made to sell the rights to her story of an alleged affair with Trump. The move was seen as a dramatic turn against Trump by the Cohen camp.
Trump's current lawyer and advisers have said he has nothing to fear from Cohen.
"If he gets indicted for something that has nothing to do with the president, well, I feel sorry for Michael, although I don't know how sorry I feel for him, because he was tape recording the world and deceiving them, including his client," Trump lawyer Rudy Giuliani told Fox News.
"But it has nothing to do with us," he added.
The case against Cohen stems in part from a referral by Special Counsel Robert Mueller, who is investigating Russian interference in the 2016 election and examined Cohen's role in at least two episodes involving Russian interests, according to people familiar with that probe.
However, Special Counsel investigators have indicated to federal law enforcement officials that the office does not require Cohen's cooperation for its probe, according to two people familiar with their work.
The investigation against Cohen first burst into public view in April, when FBI agents searched his New York office, home and hotel room. The searches - in which agents collected all of Cohen's phones and electronic devices - set off panic in the White House that federal investigators were looking into Trump's business dealings and communications with Cohen.
Since then, the probe has led to revelations about how Cohen sought to squelch negative stories about Trump and then leverage his access to the President.
After the raid, Giuliani acknowledged that the President had made a series of payments reimbursing Cohen for a US$130,000 settlement with an adult-film actress Stormy Daniels. Trump had previously denied knowledge of the payoff.
Meanwhile, leaked documents showed that Cohen was paid millions last year by companies such as AT&T and Novartis to provide advice about the new Administration.
Cohen had been under scrutiny by federal prosecutors starting in the northern autumn of 2017, when Mueller's team came across some unusual financial transactions and loans Cohen had obtained.
The Special Counsel referred the matter to the US Attorney's office for the Southern District of New York, which has been looking for evidence of possible bank fraud, wire fraud, or violations of campaign finance laws in Cohen's business dealings, according to people familiar with the matter.
The investigation has examined loans related to Cohen's taxi medallion business, and whether any laws were broken as part of an effort to stifle negative stories about Trump when he was running for president, according to people familiar with the matter.
A central focus of the probe has been on matters that have nothing to do with Cohen's most famous client, but rather Cohen's attempts to borrow millions of dollars against his taxi medallions and evidence suggesting he lied to get the money. On more than a dozen loan documents, according to two people familiar with investigators' work, Cohen dramatically inflated the value of his medallion business, year after year, even as the industry suffered from the rise of ride-sharing businesses.
In May, a New York taxi operator and former Cohen business partner agreed to cooperate with prosecutors as part of a plea deal in a separate New York state criminal tax fraud case.
Cohen is also under investigation for defrauding the IRS and failing to report his earnings, according to one person familiar with requests for information about Cohen's financial records.
Cohen worked for Trump for more than a decade, starting in 2007. The raid on Cohen's office enraged the President, who claimed prosecutors were violating attorney-client privilege.
Cohen also argued that prosecutors had violated attorney-client privilege by seizing what his lawyers said could be thousands or more items related to his work as a lawyer.
It is unusual for investigators to seize the papers of a lawyer, but in court filings federal prosecutors maintained that Cohen was doing very little legal work and that they were investigating his business dealings to search for evidence of potential crimes.
US District Court Judge Kimba Wood appointed a former federal judge to act as a special master and review the seized items to assess what material must be withheld from investigators because it is covered by attorney-client privilege.
In the end, only a tiny fraction of the seized material was found to be covered by the privilege, according to court filings.
Though Cohen has for years been portrayed as a lawyer who handed some of the most important and sensitive issues for Trump, the president has insisted to associates in recent months that Cohen was not that closely involved with him.
However, Giuliani said in May that Cohen was routinely asked to handle issues that could cause personal embarrassment for Trump, such as the claim of an affair by Daniels.
Trump has denied the affair, but Cohen directed that Daniels, whose real name is Stephanie Clifford, be paid US$130,000 just before the November 2016 election to ensure her silence, Cohen acknowledged earlier this year.
"The agreement with Michael Cohen, as far as I know, is a long-standing agreement that Michael Cohen takes care of situations like this, then gets paid for them sometimes," Giuliani said in May.
