It was an extraordinary seizure of documents that could yield an extraordinary witness against the U.S. president — his own attorney.

Former federal prosecutors say that a series of FBI raids on Monday at the Manhattan office, home and hotel room of Donald Trump's lawyer, Michael Cohen, suggest that federal officials had probable cause to believe Cohen has potential criminal exposure and that he might destroy records of his communications with clients, including Trump, unless those documents were seized.

That could spell trouble for the president, especially if it means Cohen, one of the most trusted non-family confidants in the Trump orbit, feels pressured by mounting criminal liability to "flip" and co-operate in special counsel Robert Mueller's probe into Russian meddling in the 2016 U.S. election.

"It's pretty remarkable," former federal prosecutor Barak Cohen said of the move to overcome attorney-client privilege and execute the search warrant.

"For the Department of Justice to have done this, they would have required special clearance from the U.S. Attorney ... and high-level clearance from the Department of Justice."

To preserve attorney-client privilege, legal experts say a so-called taint team of lawyers not part of the prosecutorial case in question would review the seized material to determine what prosecutors can and can't see.

Stormy Daniels, left, and Karen McDougal, right, two adult entertainers with known ties to Donald Trump, centre, received payments and signed non-disclosure agreements regarding the nature of their relationships with the U.S. president. (Matt Sayles/Associated Press, Evan Vucci/Associated Press and Dimitrios Kambouris/Getty for Playboy)

And it's likely that Michael Cohen's reported activities — involving payoffs to two women with whom Trump reportedly had extramarital affairs, a Russia-favouring "peace plan" for Russia and Ukraine, and a business deal sought for a Trump Tower in Moscow — were of interest to federal authorities.

If there's anybody who knows a lot about Trump's legal and business orbit, it's his personal lawyer, known for acting more in the capacity as a fixer, dealmaker and media attack dog for Trump than as legal counsel. It's not unheard of to execute a search warrant on an attorney's office, but executing such a warrant on the personal attorney of the U.S. president is something else altogether, perhaps even historic.

A raid on Cohen's office could yield a decade's worth of correspondence with Trump, which might explain why the president vented on Twitter on Tuesday morning:

Attorney–client privilege is dead! —@realDonaldTrump

Attorney-client privilege allows clients to discuss their legal troubles with lawyers, protecting them from being incriminated by said correspondence. But it doesn't apply under the "crime-fraud exception," which strips those protections if the communication involves an attorney and client furthering criminal activity.

In other words, the Federal Bureau of Investigation wouldn't have executed the search warrant at Trump's lawyer's office unless it had good reason and probable cause of criminality, said former federal prosecutor Renato Mariotti.

"That tells us that [the FBI] believes there wasn't any less obstructive methods to obtain the information," he said. "So they considered the privilege issues, but considered them to be outweighed by the necessity to obtain evidence that couldn't be obtained otherwise."

Trump's Deputy Attorney General, Rod Rosenstein, reportedly signed off on the search warrant. (Jacquelyn Martin/Associated Press)

Trump's Deputy Attorney General, Rod Rosenstein, reportedly signed off on the warrant. Prosecutors would have had to convince a federal judge to conclude evidence of criminal activity could be found.

The raid, first reported by The New York Times, was handled by the office of the U.S. Attorney for the Southern District of New York — not the team led by Mueller.

But Mueller did have a hand in instigating the raids. His team reportedly referred the case to the Manhattan prosecutors as a local issue. That suggests Mueller's team uncovered evidence of something criminal but tangential to their mandate of investigating Russian interference, lawyers say.

Special counsel Robert Mueller, who is investigating Russian interference in the 2016 presidential election, is said to have had a hand in instigating the raids. His team reportedly referred the case to prosecutors in Manhattan. (J. Scott Applewhite/Associated Press)

The Times reported that the documents seized in the raids related to Cohen's payment of $130,000 to the adult-film actress Stormy Daniels, who alleges she had an affair with the president, as well as records of an alleged $150,000 payment relating to Karen McDougal, a former Playboy model who also says she had an affair with Trump.

Cohen is also reportedly under investigation for possible bank fraud and campaign finance violations.

Among other items seized from Cohen were tax records, business documents, and electronic devices, according to the Times, which cited a person briefed on the search. Email communications could be of great concern if they include statements Trump made under the presumption his words would never go public.

Trump, 'certainly believes he has the power' to fire Mueller, the White House press secretary said Tuesday. Some senators fear doing so could lead to a constitutional crisis. (Alex Wong/Getty and Jim Bourg/Reuters)

While Trump railed against the raids as "an attack on our country," Michael Cohen offered a different account, telling CNN on Tuesday that FBI agents were "professional, courteous [and] respectful."

Should the president's lawyer be charged with an offence, "that could provide substantial leverage for Mueller in trying to secure his co-operation in the Russia probe," said Seth Abramson, an attorney and professor known for his popular, sometimes conspiratorial Twitter takedowns of the Trump administration.

Cohen "would be as damaging a witness against Donald Trump as any witness Mueller could secure. And that includes Trump's children," he said.

Jim Trusty, who formerly headed the Justice Department's organized crime unit, said that up until recently, Cohen was described as co-operative with the Mueller probe.

But he doubts Cohen can be compelled to betray his own client to "save his own skin."

"And an attorney who breaks the relationship between attorney and client while facing criminal charges himself is not the most likable witness," Trusty said.

In the meantime, although Mueller only made the referral to the U.S. Attorney in New York to conduct the raids, speculation was raging that Trump might try to fire Mueller. The White House press secretary said Tuesday that Trump "certainly believes he has the power to do so."

Democratic and Republican senators have warned that removing Mueller would trigger a constitutional crisis and likely lead to the president's impeachment.