The FBI has obtained a warrant needed to begin reviewing the emails found on a laptop used by Hillary Clinton aide, Huma Abedin, and her estranged husband, Anthony Weiner. The agency is reportedly set to review 650,000 emails on Weiner's laptop, which could take "weeks".

"Metadata" found on the former congressman's laptop suggests there may be thousands of emails sent to or from Clinton's private server, the Wall Street Journal reported.

FBI investigators will have to scour through the messages to determine whether they are work-related emails, duplicates from emails the FBI has already reviewed or whether they include new classified information.

FBI Director James Comey caused a political firestorm on Friday (28 October) when he informed Congress that the agency was again reviewing emails relating to the Democratic nominee's personal server.

"In connection with an unrelated case, the FBI has learned of the existence of emails that appear to be pertinent to the investigation," Comey said in a letter.

The FBI director has been criticised by Democrats and Republicans, as well as the Justice Department, for making the disclosure so close to Election Day.

Justice Department officials have said that the department has an unwritten policy of not making such investigation actions so close to an election, so they are not seen as influencing its outcome.

Officials added they were concerned about Comey's disclosure because the agency had not yet reviewed the emails. A source told ABC News: "We don't know what this is yet," adding that no one knows whether "there is any 'there' there." Despite this, Comey decided to move forward with his disclosure.

According to NBC News, Comey believed it would be better to inform Congress of the development rather than wait until after the election and risk the discovery being leaked. "In the end, we decided it was better to keep Congress informed," an unnamed FBI official said.

Agents made the discovery while investigating whether Weiner had sent illicit messages and photos to a minor. There have been conflicting reports as to whether his wife, a top Clinton aide, used the laptop to email Clinton or to print State Department emails for Clinton to read on paper.

Officials revealed that agents knew of the existence of the emails days before Comey was notified that they could be significant.

Internal note

CNN reported that Manhattan prosecutors issued a subpoena for Weiner's communications on 22 September. FBI investigators reportedly discovered the Abedin emails soon after.

In an internal note sent to FBI employees, Comey wrote he felt "an obligation" to tell Congress about the development because he had testified months ago that the investigation into Clinton's emails was completed.

"I also think it would be misleading to the American people were we not to supplement the record. At the same time, however, given that we don't know the significance of this newly discovered collection of emails, I don't want to create a misleading impression," the director's note said.

Clinton has called on the agency to reveal everything it has on the discovery.