Top FBI officials were aware for at least a month before alerting Congress that emails potentially related to an investigation of Hillary Clinton had emerged during a key stretch of the 2016 presidential campaign, according to text messages reviewed by The Wall Street Journal.

FBI Deputy Director Andrew McCabe had learned about the thousands of emails by Sept. 28, 2016, and Director James Comey informed Congress about them on Oct. 28, 11 days before the presidential election, the messages show. Mr. Comey later said nothing...