In a letter to members of Congress, the department said the announcement of the reward had not posted on its "Rewards for Justice" website because of security issues around the investigation of the attack, Voice of America reported Saturday.
The Sept. 11, 2012, attack in Benghazi killed U.S. ambassador Chris Stevens and three other Americans.
The letter, sent Friday, was in response to questions by Rep. Michael McCaul, R-Texas, chair of the House Homeland Security Committee, and other members of Congress about why rewards were not being offered for information leading to the arrest or conviction of individuals involved in the attack, CNN reported.
The department said the reward had been offered since January.
U.S. authorities have filed charges in the case, but no one has been arrested.
The case was among the FBI's "very highest priorities," FBI Director James Comey told a Senate committee Thursday.