A former staffer to Hillary Clinton has reportedly been granted immunity from prosecution, to get answers about her private email server.

The Washington Post has reported that the FBI has “secured the cooperation” of Bryan Pagliano, who set up the server in her home in New York in 2009.

The Washington Post says the information suggests that the FBI is close to winding up its investigation, prior to giving its recommendations in May.

While it couldn't secure on-the-record comment from the FBI or the Justice Department, Clinton campaign spokesperson Brian Fallon welcomed the development and said the campaign is pleased Pagliano is cooperating with the Feds.

The long-running saga of the server, which operated while Clinton was serving as US Secretary of State, has been the subject of controversy for more than a year, since it first emerged that she'd sometimes used a personal email account rather than one assigned by the State Department.

Last May, Team Hillary denied that the server had been used for any sensitive data. However, later analysis turned up messages that had been classified top-secret. Clinton's staffers said at least some of the documents were given hush-hush status by spies after she received them.

What will happen after the investigation winds up will depend on whether the FBI and DoJ think anybody has a criminal case to answer – and whether Clinton herself needs to be in the cross-hairs.

US attorney-general Loretta Lynch was neutral on that question, recently telling Congress she was waiting for recommendations from law enforcement. ®