WASHINGTON (AP) — A Republican senator is threatening to block the nomination of Chuck Hagel to lead the Pentagon unless Defense Secretary Leon Panetta testifies about the deadly assault on the U.S. diplomatic mission in Libya.

Sen. Lindsey Graham said in an interview with Fox News' "On the Record" Monday night that he would "absolutely" block the nomination until Panetta explains why the military wasn't called in during the raid.

U.S. Ambassador Chris Stevens and three other Americans died in the attack last September. Secretary of State Hillary Rodham Clinton testified for more than five hours last Wednesday before the House and Senate on the attack in Benghazi.

Graham has been critical of President Barack Obama's choice of former Republican Sen. Hagel.