Former U.S. attorneys say the FBI's raid on Michael Cohen's office is a red flag for President Trump and suggests that Robert Mueller has compiled significant evidence of criminal activity by the president's longtime personal lawyer.

These legal experts say the raid was significant because of high-profile nature of the case and the likely scope of the search warrant that authorized it.

The breadth of the search indicates prosecutors suspected Cohen possessed a trove of evidence pointing to illegal activity that he would not have turned over to law enforcement voluntarily.

But, former prosecutors note, the raid doesn't necessarily implicate Trump in criminal wrongdoing.

Instead, it may be an attempt to turn Cohen, a close confidant intimately familiar with the dealings of the Trump Organization, against the president as a key witness.

"It wouldn't surprise me if it turns out that they start to use these charges that we might otherwise think as petty or small to try to turn a very key player, the president's lawyer, to turn him and to use him as a cooperator," said Brett Tolman, the former U.S. attorney for the District of Utah, who served under the George W. Bush and Obama administrations.

Cohen is suspected of possible bank fraud and campaign finance violations related, at least in part, to his payment of $130,000 to adult-film star Stormy Daniels two weeks before the 2016 presidential election. Daniels claims to have had an affair with Trump in 2006.

Cohen created a limited liability corporation to make the payment, raising the question of whether it was an illegal in-kind contribution to the Trump campaign.

He is also linked to Trump's business dealings in Russia and reportedly reached out to the Russian government for help in gaining permits to build a high-end Trump property in Moscow. Prosecutors are also looking at Cohen's ownership of taxi medallions, according to several reports.

Cohen's attorney, Stephen Ryan, has called the FBI's search "inappropriate and unnecessary." Trump called the raid a "disgrace" and part of a "witch hunt" against him.

The president's legal team argued to a federal judge in Manhattan Friday that the documents taken by the FBI are protected by attorney-client privilege.

But former federal prosecutors say the office of the U.S. attorney for the Southern District of New York, which supervised the raid, could not have undertaken such a drastic action without extensive juridical review and compelling evidence as justification.

Raids on attorneys' offices are rare and must follow strict guidelines set by the Department of Justice's manual for U.S. attorneys.

Not only are there strict guidelines within the Department of Justice, but any judge who approves such a search is likely to demand a higher standard of justification.

The U.S attorneys' manual instructs prosecutors to "take the least intrusive approach consistent with vigorous and effective law enforcement" when seeking evidence from an active attorney.

It also mandates that any application for a search warrant for an attorney's office must have the "express approval" of the U.S. attorney overseeing the case or an appropriate assistant attorney general at Justice.

Prosecutors in the case brought the search warrant to what they call "main Justice" in Washington for sign-off because Geoffrey Berman, the U.S. attorney for the Southern District of New York, which oversaw the search of Cohen's office, recused himself from the case. Berman, who was appointed to the post in January, had donated to Trump's campaign.

Deputy Attorney General Rod Rosenstein, the most senior official at the Justice Department who is also overseeing special counsel Robert Mueller and the Russia investigation, is said to have signed off on the warrant request.

Federal prosecutors say Justice officials wouldn't have gone forward with the raid on Cohen without solid evidence, given the political firestorm it was certain to create.

Republicans in Congress questioned the tactic.

"I think that was kind of extraordinary, I didn't think that was something they should have done," Sen. Orrin Hatch (R-Utah), the former chairman of the Senate Judiciary Committee, which oversees the Justice Department, told The Hill after the raid.

Former federal prosecutors and other legal experts say that, to pursue such a warrant, the U.S. attorney's office in New York and leaders of the Department of Justice's criminal division in Washington must have had substantial reason to believe Cohen possessed incriminating documents and was likely to hide or destroy them.

"This search warrant application is likely to become the most studied and commented-on search warrant application in the history of American jurisprudence," said Timothy Purdon, a partner at the law firm Robins Kaplan who served as U.S. attorney for the District of North Dakota from 2010 to 2015.

"The amount of evidence they must have had to present must have been overwhelming, really substantial," he said.

Purdon was appointed U.S. attorney during the Obama administration.

Matthew Orwig, who was appointed by President George W. Bush to serve as U.S. attorney for the Eastern District of Texas, said the U.S. attorney's office for the Southern District of New York would not have proceeded with the raid without strong evidence of criminal wrongdoing.

"For a lawyer's office, they would be particularly vigilant to make sure they had justifiable reason for doing that," he said.

He said the standard would be even higher for the personal attorney of the president of the United States.

"It's a very, very high standard," he said.

Orwig says he remembers only a few times during his 15-year career as a prosecutor when his team felt compelled to gain a search warrant of a fellow attorney's office.

He said Mueller's legal team would likely not have referred the matter to the U.S. attorney for the Southern District of New York unless they strongly suspected Cohen of criminal wrongdoing.

"I don't think there's any doubt that a grand jury is looking at him seriously," Orwig said.

The U.S. attorneys' manual also states that search warrants must be as specific as possible.

The FBI's decision to search Cohen's office in Rockefeller Center as well as the Park Avenue hotel room where he was living indicates they suspected a wide array of incriminating documents spread in various locations, prosecutors say.

"The fact that they were able to go into his home and his office and all of these areas, tells me there was probably quite a bit in that probable cause statement," said Tolman, the former U.S. attorney for the District of Utah.

Former U.S. attorneys appointed by Bush and Obama also dispute the characterization by Trump's lawyers that the FBI raid necessarily violated attorney-client privilege.

While some of the documents seized may have been protected, there are safeguards in place at the Justice Department to prevent prosecuting attorneys from seeing them.

A so-called "taint team" will screen the Cohen evidence before any of it is given to prosecutors. It will be up to a judge to sort out which documents are covered by attorney-client privilege and which are not.

"Any time you search a lawyer's office you're going to have attorney client privilege but that doesn't mean that every single document is covered by attorney client privilege," Orwig said.

Legal experts said the privilege only covers a lawyer offering legal advice but does not extend to criminal activity.

"Lawyers can commit crimes and if they do, they're subject to search warrants just like anybody else," said Purdon.

In addition, Cohen's services for Trump since he became involved with his business empire in 2006 extended well beyond providing legal advice.

"Apparently Michael Cohen did a whole lot of stuff for Donald Trump that had nothing to do with being a lawyer," said Caroline Fredrickson, president of the American Constitution Society, a progressive legal advocacy group.

She said Cohen was reportedly involved of a number of deals for Trump that "didn't involve giving legal counsel," citing his pursuit of business deals in Russia.

"There's no reason that kind of work or conversations about that with Donald Trump would be protected or be covered any kind of attorney-client privilege."

The government made a similar point in a court filing Friday, asserting that Cohen has "a low volume of potentially privileged information" because he is primarily a businessman, not a lawyer.