Dempsey married Margo Dempsey sometime prior to 2386. Like him, Margo was also an FSA officer. Following the ouster and arrest of Baras Rodirya from the office of Federation President Pro Tempore, Baras was turned over to the Bajorans to face charges for his collaboration with the Cardassians. Dempsey was assigned as to a protection detail for a Federation diplomat who was going to try to convince the Bajorans to extradite Baras to the Federation to stand trial.
For some reason, Uraei did not want this diplomat to make it to Bajor to request the extradition of Baras. It had Section 31 engineer the destruction of the transport this diplomat and Dempsey were on board, killing both of them and everyone else on the transport. Section 31 made it appear that this had been an unfortunate accident.
When Ozla Graniv exposed Section 31's activities to the public, Margo learned Dempsey'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 connection to Dempsey nor her obvious conflict of interest to the FSA when it began assigning officers to guard the 31 agents they had arrested.
As a result, Margo was assigned to guard Starfleet Admiral William Ross when he was taken to meet with Federation Attorney General Phillipa Louvois. Seeing her chance, during the meeting Margo fired a phaser set to kill at Ross and his attorney Captain Rebecca Steeby, killing both. She nearly killed Louvois as well but Louvois was able to deflect her shot long enough for her fellow FSA agent to stun her with his phaser.
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)