From Halopedia, the Halo wiki
July 18, 2552
UNSC Marine Corps
Lieutenant McCasky was a member of the UNSC Marine Corps in 2552.
He was a commanding officer groundside during the Battle of Sigma Octanus IV on a scouting mission with his squad. McCasky had underestimated the danger of the mission, assuming it was a "milk run", when he and most of his Marines were killed near their camp by a pair of Mgalekgolo.
Four members of his unit did survive though, Corporal Harland, Private Fincher, Private Walker, and Private Cochran, who escaped in a Warthog.