Melanie Eversley

USA TODAY

The former State Department staffer who set up Hillary Clinton's private email server in her home has been granted immunity, the Washington Post reports it has learned from an unnamed senior federal law enforcement official.

Bryan Pagliano worked on the Democratic presidential hopeful's 2008 White House campaign and set up the unsecured server in 2009, according to the Post.

FBI investigators are likely to want to know why Clinton decided to use a private email server while she was U.S. secretary of state, as well as whether other users were aware they were communicating via a private server, the Post reports.

Clinton was secretary of state from 2009 to 2013.

The development comes as Clinton wages a tense campaign for the Democratic White House nomination against Bernie Sanders. On Tuesday, she logged seven primary wins compared to four by Sanders. Her opponent raised $42.7 million in February - the largest one-month draw for any candidate in 2016 - compared to $30 million Clinton generated in the same time period.

Hillary Clinton collects $30 million in February

Last month, the Obama administration announced that 22 emails that passed through that private server were top secret and would not be released to the public.

37 pages of Hillary Clinton emails withheld as 'top secret'

Before the top secret designation, Clinton maintained she did not use that server for any classified material. She has since apologized for the email mix-up.

The Hillary Clinton campaign could not be reached late Wednesday, but Brian Fallon, campaign press secretary, said via Twitter that the campaign is pleased. Fallon made reference to Pagliano's decision last September to invoke his Fifth Amendment rights in his appearance before a U.S. House committee investigating the 2012 attack on a U.S. compound in Benghazi, Libya.

"We disagreed w/his decision not to answer questions from Benghazi Committee so are pleased he is cooperating now," Fallon tweeted.

Justice Department spokesman Marc Raimondi said Wednesday night the agency had no comment on the Post piece.