House Republicans have called FBI Director James Comey to testify about Hillary Clinton's use of private email as secretary of state.
On Tuesday, the FBI recommended Mrs Clinton, the likely Democratic presidential nominee, should not face criminal charges in the matter.
However, Mr Comey said Mrs Clinton and her staff were "extremely careless" in handling classified materials
Over 100 classified messages were found on her email servers, the FBI said.
Mrs Clinton had previously said she had not knowingly sent or received classified material from her private email account.
Mr Comey will testify on Thursday before the House Committee on Oversight and Government Reform, led by Republican Jason Chaffetz.
"The FBI's recommendation is surprising and confusing," Mr Chaffetz said.
"The fact pattern presented by Director Comey makes clear Secretary Clinton violated the law," he said. "Individuals who intentionally skirt the law must be held accountable."