The paparazzi lingering outside the Loews Regency on Park Avenue, hoping to get a photo of U.F.C. fighter Conor McGregor, appeared not to notice the stream of F.B.I. agents that entered the New York hotel early Monday morning, as they made their way up to the room where the president’s personal lawyer, Michael Cohen, has been staying. A handful of them remained upstairs for several hours, according to a source familiar with the situation. “Today the U.S. Attorney’s Office for the Southern District of New York executed a series of search warrants and seized the privileged communications between my client, Michael Cohen, and his clients,” Cohen’s attorney, Stephen Ryan, said in a statement. “I have been advised by federal prosecutors that the New York action is, in part, a referral by the Office of Special Counsel, Robert Mueller.”

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Downstairs, hotel jazz blared on speakers, as women with Louis Vuitton totes and men in Gucci loafers and gold-button blazers checked in and out, while kids in Yeezys and fur-trimmed puffer coats chased each other around the lobby. A woman who looked awfully like Caroline Kennedy made a quick pass through the restaurant, and hotel security seemed to arrange a way for a guest, perhaps McGregor, to make an exit through a side door without being spotted.

The cheery hotel scene belied what has become a growing legal headache for Cohen, as Mueller’s Russia probe has expanded to include inquiries into the Trump Organization’s business records and foreign dealings, including in Russia. Last week, McClatchy reported that the special counsel’s investigators showed up with subpoenas to the home of an associate of the Trump Organization, compelling sworn testimony and electronic records. The report indicated that investigators were interesting in interactions involving Cohen.

Later Monday, The Washington Post reported that Cohen is under federal investigation for possible bank fraud, wire fraud, and campaign-finance violations. Cohen’s home and Manhattan office were also raided.

Cohen’s lawyer called the tactics “completely inappropriate and unnecessary,” and said the documents seized concerned protected attorney-client communications. “These government tactics are also wrong because Mr. Cohen has cooperated completely with all government entities, including providing thousands of non-privileged documents to the Congress and sitting for depositions under oath.”

In brief comments to the White House press pool Monday afternoon, Donald Trump railed against the treatment of his personal attorney, calling the raid “a disgraceful situation” and repeating his assertion that the Mueller investigation is a witch hunt. “It’s an attack on our country . . . what we all stand for.”

A spokesman for the special counsel’s office declined to comment on Monday, as did the F.B.I.’s New York office. Cohen’s cell phone, which is typically ringing off the hook, went straight to voice mail all day on Monday. The New York Times separately reported that the warrant was related to payments Cohen made to adult-film actress Stormy Daniels, among other issues.

Cohen, a loyal fixer for the Trump Organization and longtime personal friend of the Trump family, has been a person of interest in the Mueller probe since 2016, when his name surfaced in the Trump-Russia dossier compiled by Christopher Steele—a controversial opposition-research document that included claims that Cohen had traveled to Prague to meet with Russian operatives to “clean up the mess.” (Cohen has told me repeatedly that he has never been to Prague, and that the claims in the dossier are untrue; he filed a defamation suit earlier this year against Fusion GPS, which commissioned the dossier, and BuzzFeed, which published it, seeking $100 million in damages.) Last year, it was reported that Cohen had been working on a plan to build a Trump Tower in Moscow, which did not materialize. In September, he was questioned by congressional committees over the course of two days as part of their investigation into whether the Trump campaign colluded with Russia.

More recently, Cohen’s relationship with the president has become headline news because of the payment he made to Daniels 11 days before the 2016 election. In January, The Wall Street Journal reported that a shell company set up by Cohen had paid Stephanie Clifford, who goes by the stage name Stormy Daniels, $130,000 as part of a non-disclosure agreement barring her from discussing her allegation of a 2006 affair with Trump. Her attorney has since filed a lawsuit claiming that the agreement is void because Trump did not sign it; attorneys for Cohen have filed their own suit attempting to bring the matter into arbitration. Trump, who has denied the affair through spokespeople, said aboard Air Force One on Thursday that he did not know about the agreement—a contention Cohen has repeatedly made as well. While ethics experts have suggested that the payment could represent an illegal campaign contribution if it were deemed to have been made for electoral purposes, Cohen has repeatedly told me that the payment came out of his own pocket and that it had nothing to do with the election. “What I did defensively for my personal client, and my friend, is what attorneys do for their high-profile clients,” Cohen told me last month. “I would have done it in 2006. I would have done it in 2011. I truly care about him and the family—more than just as an employee and an attorney.”

The arrival of F.B.I. agents at the Regency marks another dramatic escalation of the Mueller probe. While Trump’s allies have dismissed many of the people in the special counsel’s crosshairs as low-level campaign officials, or waved away the charges against Paul Manafort as unrelated to Trump, Cohen is inextricably and emotionally linked to the Trump family. He worked in Trump Tower every day, and still remains one of Trump’s personal attorneys, though he left the Trump Organization around the presidential inauguration. Last summer, he told me he would “take a bullet for the president.”

For months after Mueller’s appointment, Trump and Cohen did not speak regularly, at the advice of counsel, but earlier this year, they started to communicate more. He has had dinner twice at Mar-a-Lago in recent months, including a meeting with Trump on the eve of Stormy Daniels’s sit-down on 60 Minutes.

This article has been updated.