One of Hillary Clinton's key aides who helped maintain her private e-mail server is defying a Congressional subpoena and won't testify at a hearing. He faces arrest.

Bryan Pagliano, a former staffer on Clinton's 2008 presidential bid who was granted immunity by the Justice Department during a probe of the server, has twice before invoked his Fifth Amendment rights on the matter.

The latest call to testify furthered "no legislative purpose and is a transparent effort to publicly harass and humiliate" him "for unvarnished political purposes," Pagliano's lawyers wrote in a letter to the committee's chairman, Republican Representative Jason Chaffetz of Utah. Lawmakers can jail people who defy congressional subpoenas.

The FBI director said Democrat presidential candidate Hillary Clinton had been 'extremely careless' in her handling of US classified information.

Pagliano installed the main server that Clinton set up in her suburban New York home while she was secretary of state, and he handled software upgrades and general maintenance, according to recently released files from the FBI's investigation, which recommended no charges against Clinton or her aides.

Her campaign for the White House has been repeatedly dogged by that move, which has prompted a series of attempted explanations and clarifications from the Democratic presidential candidate. Two months ago, the FBI director said she had been "extremely careless" in her handling of sensitive government information.