The inquiry is an attempt to gather additional information about how many sensitive documents were sent via Clinton's private server during her tenure as secretary of state and who had knowledge of the practice, The Daily Beast reported citing an "ally" of US Attorney General Jeff Sessions familiar with the issue.
Moreover, the Justice Department's prosecutors are inquiring whether Clinton’s staffers, including her top aide Huma Abedin, made immunity agreements with US law enforcement during the initial investigation.
The FBI closed its criminal investigation in July 2015 after the FBI concluded that Clinton did not intend to violate US laws and recommended no charges should be filed against her. However, the FBI acknowledged that Clinton was "extremely careless" in handling her email communications and sent confidential emails on numerous occasions.
The FBI briefly reopened the investigation prior to the 2016 US presidential election due to newly discovered emails that may be pertinent to the case, but then closed the second probe and said it stands by its earlier conclusion not to criminally prosecute Clinton.
Clinton used a private server and email accounts for official business during her tenure as secretary of state from 2009 to 2013, contrary to US regulations and established practice.