Even his co-hosts did not know until 4 a.m. on Wednesday, just hours before the official announcement, that Matt Lauer had been fired after a serious allegation of sexual misconduct. That was when the NBC News president, Noah Oppenheim, called two of his “Today” show anchors, Savannah Guthrie and Hoda Kotb.

They would be broadcasting live to millions of viewers in three hours — and Mr. Lauer would not be joining them.

Senior executives at NBC News made the decision to fire Mr. Lauer, the face of “Today” for two decades, late Tuesday night. At 6:49 a.m., 11 minutes before airtime, Andrew Lack, the news division’s chairman, sent a memo to the staff.

In the note, he referred to “a detailed complaint from a colleague about inappropriate sexual behavior in the workplace by Matt Lauer.” Mr. Lack added that there was “reason to believe this may not have been an isolated incident.”