Standish

Major Standish was an ONI operative. When the crew of the UNSC vessel Apocalypso discovered a deep space Forerunner Artifact (presuming it being a Covenant Weapon, Major Standish tried to cover it up so that he could take the glory of uncovering whatever secrets the artifact would hold.

When Colonel Herzog started threatening to expose Major Standish for keeping the artifact from his superiors, Standish had him assassinated. Durga, who had been spying on Herzog, hacked into the Naval computer networks and exposed his severe breach of protocol to his superiors when Herzog's agent, Rani Sobeck was arrested. Later, an agent codenamed Sierra-1085 was executed for the assassination. It is highly likely this is Standish, but it could also be one of his underlings.