At 7:57 p.m., F.B.I. employees received an email from Attorney General Jeff Sessions officially announcing their new boss. “The president of the United States has exercised his lawful authority to remove James B. Comey Jr. as the director of the Federal Bureau of Investigation,” the email read.

“By operation of law and effective immediately, deputy director Andrew McCabe assumed the position of acting director of the F.B.I. As you well know, the F.B.I. is an exceptional law enforcement and intelligence agency. It is made so by you, the devoted men and women who work tirelessly to keep our country safe. Thank you for your steadfast dedication and commitment during this time of transition.”

— Rebecca R. Ruiz

‘Why now?’

Senator Chuck Schumer of New York, the Democratic leader, said Mr. Trump had called him earlier on Tuesday to inform him of the firing.

“I simply said to him, ‘Mr. President, in all due respect, you’re making a very big mistake,’” Mr. Schumer recalled at a hastily arranged news conference Tuesday evening. “And he didn’t really answer.”

Mr. Schumer said the first question the administration must answer was, “Why now?”

“This investigation must be run as far away as possible from this White House and as far away as possible from anyone President Trump has appointed,” he said of the inquiry into ties between Mr. Trump’s associates and Russia.

He called for a special prosecutor to investigate those ties and said that firing Mr. Comey continued a “deeply troubling pattern” from the administration, citing the removals of Sally Q. Yates as acting attorney general and Preet Bharara as United States attorney in Manhattan.

— Matt Flegenheimer

Trump accuses Schumer of a double standard

Mr. Trump took to Twitter late Tuesday to respond to Mr. Schumer’s criticism, citing a comment Mr. Schumer made in early November, shortly after the F.B.I. briefly reopened its inquiry into Hillary Clinton’s use of a private email server.