In July, after a year-long investigation, the FBI concluded that charges against Clinton were not warranted, saying agents had not found any evidence that Clinton knowingly sent or received classified information on the server.
On October 28, Comey wrote to Congress that the FBI was going to investigate emails potentially related to Clinton that had been found during an unrelated investigation.
"Since my letter, the FBI investigative team has been working around the clock to process and review a large volume of emails from a device obtained in connection with an unrelated criminal investigation," Comey wrote today. "During that process we revirewed all of the communications that were to or from Hillary Clinton while she was Secretary of State.
"Based on our review, we have not changed our conclusions that we expressed in July with respect to Secretary Clinton," he wrote.
Clinton spokesman Brian Fallon said the campaign never doubted that the new investigation would come to the same determination as the first did.
"We were always confident nothing would cause the July decision to be revisited," Fallon tweeted. "Now Director Comey has confirmed it."
The bureau team, which previously investigated Clinton's use of a private email server while she served as secretary of state, found the emails on a hard drive pulled from a laptop used by Clinton aide Huma Abedin's estranged husband Anthony Weiner, the former New York congressman who is under investigation for allegedly sexting with a minor.
Sources told ABC News tonight that the new batch of emails contained duplicates the FBI had already reviewed and did not contain new information that altered their initial findings. Agents were aided by computer programs that sorted through thousands of emails found on the laptop shared by Abedin and Weiner.
Comey received heavy criticism for his decision to notify Congress so close to the election that his agency was reviewing the emails after previously telling lawmakers the investigation into Clinton’s server had been completed.
In a Washington Post editorial, former Attorney General Eric Holder said Comey broke with department policy of not taking actions close to Election Day, and that his actions “ran counter” to guidance he instituted four years ago for ongoing investigations during election season.
Holder was also among nearly 100 former Justice Department officials who signed on to an open letter, saying that Comey's letter to Congress was inconsistent with prevailing department policy, that it broke with longstanding practices followed during past elections and that they were astonished and perplexed" by his action.