The official in Deputy Attorney General Sally Q. Yates’s office who received the FBI call immediately understood the explosive potential of Comey’s message, coming so close to the presidential election. Federal attorneys scrambled into offices on the fourth and fifth floors of Justice Department headquarters, where they huddled to figure out how to stop what they viewed as a ticking time bomb.

“It was DEFCON 1,” said an official familiar with the deliberations. “We were in­cred­ibly concerned this could have an impact on the election.”

Aides at Justice and the FBI — located in offices directly across the street from each other on Pennsylvania Avenue — began exchanging increasingly tense and heated phone calls, nearly a half-dozen throughout the afternoon and evening of Oct. 27 and into the next morning.

Justice officials laid out a number of arguments against releasing the letter. It violated two long-standing policies. Never publicly discuss an ongoing investigation. And never take an action affecting a candidate for office close to Election Day. Besides, they said, the FBI did not know yet what was in the emails or if they had anything to do with the Clinton case.

Remarkably, the country’s two top law enforcement officials never spoke. As Comey’s boss, Attorney General Loretta E. Lynch could have given the FBI director an order to not send the letter. But Lynch and her advisers feared that Comey would not listen. He seemed to feel strongly about updating Congress on his sworn testimony about the Clinton investigation. Instead, they tried to relay their concerns through the Justice official whom the FBI had called.