FBI Obtains Warrant Needed to Start Reviewing Emails Found on Laptop Used by Clinton Aide Emails were on laptop used by Huma Abedin and husband Anthony Weiner.

 -- Federal investigators looking into the Hillary Clinton email matter have obtained a warrant needed to start reviewing the emails found on a laptop used by Clinton aide Huma Abedin and her husband, Anthony Weiner.

FBI Director James Comey told Congress Friday that the agency was again reviewing emails related to Clinton's personal server after it learned of additional information that might be relevant to the case.

He sent a letter to key members of Congress informing them that although the FBI previously announced the completion of its investigation into her email server, new information prompted additional review.

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

Investigators came across the emails while looking into a completely separate criminal matter, former Rep. Anthony Weiner's alleged sext messages to an underage girl in North Carolina. Weiner, a Democrat from New York, is married to but has separated from Huma Abedin, one of Clinton's most trusted aides, and investigators found what Comey described as "emails that appear to be pertinent" to the Clinton probe on at least one electronic device used by Weiner and Abedin.

According to a law enforcement source familiar with the investigation, federal authorities looking into the Weiner matter discovered the emails weeks ago. The process that followed was a complicated one, with lots of back-and-forth over how to handle the situation and whether the FBI needed to get a warrant in order to review the newly discovered emails.

The Clinton email investigators put together a summary of the situation and presented it all to Comey on Thursday, after which he agreed that the Clinton email team should seek a warrant to review the newly discovered emails.

In his letter, Comey said the review would be carried out "to determine whether they contain classified information, as well as to assess their importance to our investigation."

"We don't know the significance of this newly discovered collection of emails," he wrote to FBI personnel in an internal message later in the day.

It's unclear how long the review process will take, according to Comey.

During a House hearing last month, he was asked if he would reopen the Clinton investigation if "relevant and substantial" information was uncovered. He responded, "It's hard for me to answer in the abstract. We would certainly look at any new and substantial information."

In conversations with FBI counterparts hours before Comey disclosed that the FBI was hoping to review the emails, Justice Department officials emphasized that the department has long steered clear of taking such investigative actions close to an election if those actions could influence the outcome of an election, sources familiar with the matter told ABC News.

Justice Department officials were also concerned about the planned disclosure because the FBI had yet to review the newly discovered emails.

"We don't know what this is yet," one source said, adding that no one knows if "there is any there there."

But Comey "decided to move forward anyway," another source said.

After Comey told Congress of the decision in a letter Friday, he said he felt compelled to do so because he previously assured lawmakers and the public that the FBI investigation was completed.