Even after Comey received the desired information, major questions still remain — for instance, how many emails are related to Clinton or contain classified information. Since notifying Congress, Comey has drawn intense criticism from lawmakers in both parties as well as prominent former law enforcement officials for publicizing the investigation so close to the election when so little was known.

It is unclear what FBI agents have learned since discovering the emails in early October. But officials say they gained enough information from the email metadata to take the next step, seeking a warrant to review the actual emails. That legal step prompted Comey’s letter to Congress, which has made him a central figure during the stretch run of the presidential campaign.

“He needed to make an informed decision, knowing that once he made that decision, he was taking it to another level,” an official with knowledge of the decision-making process said.

Law enforcement officials on Oct. 3 seized the computer belonging to Weiner, who was under investigation for allegedly sending suggestive online messages to a teenage girl. As they examined his computer, investigators quickly stumbled on emails tied to Abedin. She and Weiner separated in August. Abedin, like Clinton, used an email address that was routed through Clinton’s private server.