Vault 79 was constructed in secret in order to remove the United States federal gold reserve from Fort Knox and hide it where it would be safe from Chinese nuclear strikes. The vault was located near the town of Berkeley Springs, and Montgomery and her fellow soldiers regularly drank in the town's bars.
Kathryn and a group of fellow soldiers who were stationed at Vault 79 stole at least 20 million dollars from the reserve before deserting to gamble on the Las Vegas Strip. Desperate to keep Vault 79 a secret, on Wednesday, July 11th, 2072 the government had the Las Vegas police and United States Army engage the deserters in the Ultra-Luxe casino, where despite their attempts to surrender they were all killed, posthumously being declared mobsters and the shootout portrayed as a heavy blow to the city's organized crime.
Kathryn wasn't present at the Ultra-Luxe, having been staying at the Lucky 38, where she was found the next day, with duffle bags containing what was left of the money the group had stolen from the reserve. Branded as a Chinese spy called "Shanghai Sally," she escaped her hotel room using a bedsheet rope, fleeing east through Flagstaff, Albuquerque, and El Paso, stealing several vehicles, allegedly including an Army vertibird, over her four days on the run. Early on Sunday, she arrived in Galveston, Texas, where she was recognized by a Red Rocket attendant. Stealing a speedboat, she attempted to flee into the Gulf of Mexico, but her boat was fired upon and destroyed by the USS Wade.
Her death then became the focus of an intense cover-up, with all evidence of Sergeant Montgomery and her group's real identities being destroyed, with Federal authorities questioning the residents of Berkeley Springs, near Vault 79's hidden location, and kidnapping anyone who knew the group wasn't who the media were painting them to be, including Kathryn's partner C. Rutkowski.
The cover-up attracted the attention of Flavia Stabo, who uncovered the truth of who Montgomery's group were, and where they were stationed. Stabo didn't figure out where exactly the money came from, dubbing it the Treasure of Appalachia, and was killed by Vault 79 security trying to investigate the vault entrance.