Having married sometime prior to 2386, and Dempsey's husband Clark was also a member of the FSA. Clark Dempsey had died in a shuttle crash when Section 31 took out the shuttle in order to prevent a Federation diplomat from reaching Bajor to argue for the extradition of Ishan Anjar to face Federation justice.
When Ozla Graniv exposed Section 31's activities to the public, Dempsey learned that her husband's death was not an accident. Deciding to take revenge on 31's agents for his death, she began manipulating her record so that the background checks would not reveal her husband's death to the FSA when assigning officers to guard the 31 agents they had arrested.
As a result, Dempsey was assigned to guard former Starfleet Admiral William Ross when he was taken to meet with Federation Attorney General Phillipa Louvois. Seeing her chance, during the meeting Dempsey fired a phaser set to kill at Ross and his attorney Captain Rebecca Steeby, killing both.
Following their deaths, Dempsey was taken in to custody, with Admiral Leonard James Akaar expecting her to be tried and convicted on charges of murder. The FSA and Starfleet revamped security procedures in order to keep Section 31 agents in custody from being attacked by any more of 31's vengeful victims or from 31 agents still free. (TNG novel: Available Light)