A former US state department employee who set up the email server used by Hillary Clinton when she was secretary of state has been granted immunity, according to US media reports.
It comes as part of a criminal investigation into her possible use of the server for classified emails.
Mrs Clinton denies doing anything wrong, in a case that has dogged her US Democratic presidential campaign.
Thousands of emails have been released except for those deemed “top secret”.
In January, the Department of State said about 2,000 emails contained classified information, including 22 classified as “top secret”, its highest level. Hillary Clinton, who served as secretary of state from 2009 to 2013, has since apologised for using a personal email account for official business during that time.
But she has continued to deny that she had broken any government rules or laws, saying none of the material sent via email had been marked classified at the time.
According to the Washington Post, quoting a senior law enforcement official, the FBI is working with Bryan Pagliano, a former 2008 presidential campaign aide who helped set up the server in 2009, in the criminal case.
Brian Fallon, a spokesman for Hillary Clinton, said he was “pleased” Mr Pagliano was cooperating with prosecutors.
The email saga has at times overshadowed Mrs Clinton’s 2016 run for the White House. However, she is leading the Democratic race for presidential nominations, after sweeping up seven more states in primary and caucus voting on Super Tuesday this week.