Thumb drives containing copies of emails Hillary Clinton sent and received while serving as US secretary of state are now in the possession of the FBI, media reports say.
Multiple outlets also reported Clinton has directed her staff to hand over the personal email server she used.
"It is her hope that State and the other agencies involved in the review process will sort out as quickly as possible which emails are appropriate to release to the public, and that the release will be as timely and transparent as possible," Clinton campaign spokesman Nick Merrill told CBS News.
"She directed her team to give her email server that was used during her tenure as Secretary to the Department of Justice, as well as a thumb drive containing copies of her emails already provided to the State Department."
Clinton has been criticised for her decision to use a personal email server while secretary of state, and the Federal Bureau of Investigation has recently begun looking into the possibility that Clinton sent and received classified information from the server.
Last month, the inspector general of the US intelligence community found that four emails sent through that server - in a sample of 40 of the 30,000 emails provided by Clinton - contained information that was classified when they were sent.
Earlier Tuesday (local time), the inspector general's office sent a letter to Senator Charles Grassley, head of the US Senate's Judiciary Committee, informing him that it had determined two of those four emails were, in fact, highly classified - meaning the information they contained was more secret than originally thought.
This determination appeared to trigger the handing over of the emails to the FBI Tuesday.
The State Department has disputed whether the emails in question were classified at the time they were sent.
"Department employees circulated these emails on unclassified systems in 2009 and 2011 and ultimately some were forwarded to Secretary Clinton. They were not marked as classified," State Department spokesman John Kirby said on Tuesday.
The thumb drives had been in the possession of Clinton's attorney, David Kendall, but were turned over to the FBI after the agency determined he could not keep the classified information they are thought to contain, according to US media reports.