The US State Department is offering a $10 million reward to track down anyone behind last year's brutal attack on a US mission in Libya, in which the ambassador was among four Americans killed. "Since January of this year, the Rewards for Justice program has had a reward offer of up to $10 million for information leading to the arrest or conviction of anyone who was involved in the September 2012 Benghazi attacks," a State Department spokesman told AFP on Friday.