A former U.S. State Department information technology staffer who worked for Hillary Rodham Clinton is refusing to testify before U.S. lawmakers probing the former top diplomat and the 2012 attacks on U.S. installations in Benghazi, Libya, according to a letter sent by his lawyer to congressional investigators.

Clinton, the front-runner for the Democratic presidential nomination, was secretary of state at the time of the attacks, in which four Americans were killed. A Republican-led House of Representatives Select Committee is investigating the incident as well as Clinton's use of a private email server during her tenure.

Mark MacDougall, a lawyer for former Clinton aide Bryan Pagliano, said in a letter sent on Monday to committee chairman Trey Gowdy that the committee had asked his client to testify and to produce documents related to "servers or systems owned personally and/or controlled by former Secretary of State Hillary Clinton from 2009 through 2013."