Mr. Kasowitz’s spokesman, Michael Sitrick, has said the ProPublica story was false. But on Wednesday night, the unidentified man sent Mr. Kasowitz’s law firm an email with the subject line “Resign Now.”
The email writer wrote that while he did not know Mr. Kasowitz, he believed that “no good can come” from the lawyer’s efforts to defend Mr. Trump.
Mr. Kasowitz responded with four obscenity-laced emails in 33 minutes, according to ProPublica. He gave a phone number, demanded that the man call him and told him to “watch his back.”
“I already know where you live, I’m on you,” the last email said. “You will see me. I promise.”
ProPublica said the man was so unnerved by the threat that he contacted the F.B.I.
Mr. Sitrick said on Thursday that Mr. Kasowitz intended to apologize to the email writer. “While no excuse, the email came at the end of a very long day that at 10 p.m. was not yet over,” Mr. Sitrick said.
He quoted Mr. Kasowitz as saying, “The person sending that email is entitled to his opinion, and I should not have responded in that inappropriate manner.” Mr. Kasowitz added, “This is one of those times where one wishes he could reverse the clock, but of course I can’t.”
There were indications early this week that Mr. Kasowitz, one of four lawyers on the president’s legal defense team, was deeply frustrated by the failure of senior White House advisers to include him in discussions related to the Russia inquiry, and might quit. But in the past two days, people familiar with the situation have said he intends to remain on board.