In the last hour of Day 3 she arrived at CTU to pick up her husband's things. She was highly distraught at her loss as she spoke to Michelle Dessler (Gael requested Michelle to tell his family that he had died a quick and painless death, as he refused to be euthanized, and Michelle complied when speaking with Theresa).
When Michelle left Gael's workstation, Theresa was being watched by Barry, but when he was distracted she was able to take a gun that was in Gael's desk. Using the gun, she shot Stephen Saunders as he was trying to identify Arthur Rabens. She knew he was the one to blame for Gael's being infected with the Cordilla virus. She was apprehended for the crime at CTU.