The U.S. State Department revealed Friday that it has been offering a reward of up to $10 million for information leading to the arrest or conviction of any individual involved in last year's terror attack on an American diplomatic facility in Benghazi, Libya.

In a letter sent to lawmakers on Friday, the department said the rewards were not publicized on its "Rewards for Justice'' website as is normally done because of security issues around the ongoing investigation into the Sept. 11, 2012 attack in Benghazi. The U.S. ambassador to Libya, Chris Stevens, and three other Americans were killed.

U.S. lawmakers had complained the department was not using everything at its disposal to catch the perpetrators. Several lawmakers last month sent a letter to Secretary of State John Kerry asking why rewards were not being offered for the Benghazi attackers.   
State Department officials said the rewards had been in place since January, while Hillary Clinton was still secretary of state.