As Steve investigates further, he teams up with Natasha, Sam Wilson (Anthony Mackie), and Brock Rumlow (Frank Grillo) to uncover the truth. They learn that Alexander Pierce (Robert Redford), a high-ranking S.H.I.E.L.D. official, is working with HYDRA, a terrorist organization thought to be eradicated. The Winter Soldier, a brainwashed Bucky Barnes (Sebastian Stan), Steve's best friend from World War II, is revealed to be the assassin carrying out HYDRA's missions.