Hillary Clinton
Hillary Clinton has been embroiled in an ongoing email scandal as she vies for the Democratic presidential nomination.Reuters

The US Department of Justice has granted immunity to a former Hillary Clinton staff member, who worked on her private email server during her time at the State Department in the ongoing criminal investigation into the potential mishandling of classified information. A senior law enforcement official said Bryan Pagliano, who worked on Clinton's 2008 presidential campaign before setting up her server in 2009, was cooperating with the FBI.

According to The Washington Post, the FBI is likely to want to interview the Democratic presidential frontrunner and her senior aides before concluding the investigation in the coming months. The bureau will look into how the private server was set up, why it was decided that a private server should be used and whether the staffers knew they were sending classified information in emails.

As of yet, there are no signs that prosecutors have convened a grand jury to subpoena testimony or documents, The Post reported. Neither the FBI nor Justice Department discussed the investigation and Pagliano's attorney Mark J MacDougall declined to comment to the Post.

"As we have said since last summer, Secretary Clinton has been cooperating with the Department of Justice's security inquiry, including offering in August to meet with them to assist their efforts if needed," said Clinton campaign spokesman Brian Fallon. He added the campaign was "pleased" Pagliano, who previously invoked his Fifth Amendment rights before a congressional panel, was cooperating with prosecutors.

The former secretary of state has apologised for using a private server for official State Department business. "Yes, I should have used two email addresses, one for personal matters and one for my work at the State Department. Not doing so was a mistake. I'm sorry about it, and I take full responsibility."

Clinton served as secretary of state from 2009 to 2013. The investigation into her private email server comes as the former senator goes head-to-head in a fierce competition against Vermont Senator Bernie Sanders for the Democratic presidential nomination. Clinton, who won seven primary wins on Super Tuesday, is well on her way to earning enough delegates to secure the nomination.