Sigma Squad was a UTA Marines squad led by Winifred McCall.
Sigma Squad was sent to quell the violence in Hangar 4, when the afflicted there began to riot and murder each other. They went in with hazmat suits, so as to not contract the Phobos virus. When they entered the hangar, they found the dismembered bodies of twenty refugees, eleven males and nine females, spelling out HELP US. They continued forwards and found four afflicted, three men and a woman about to kill a girl of ten. The afflicted were then killed, but the girl turned out to also be afflicted by Phobos and attacked Sergeant McNulty, injuring him and ripping his hazmat suit. Corporal Sykes then took her out. However, the squad then came under attack from the remaining afflicted, who threw pipe bombs at them and attempted to cut off their retreat, but were taken out by several pulse grenades. They retreated with heavy losses. Sigma was then forced to abandon Sergeant McNulty because of his wound.
Sigma Squad was then ordered to bring Byron Zhang and Consuela Nestor from the Hypatia to the Alexander. After a brief standoff, a Kerenzan recruit, Doherty mistook Ann Chau's surrender for an attack on McCall. Chau was then killed, but Zhang and Nestor were brought over to the Alexander.