Ensign Keller was a Starfleet officer serving on board the USS Enterprise-D. She was stationed in main engineering, and was killed in an accident sometime between 2366 and 2368. Engineering personnel needed to bring down the isolation door between engineering and the warp core, but Keller did not escape before Commander Riker ordered the door to be sealed.
Riker experienced some guilt and trauma related to the incident. In mid 2368, the Ullian Jev caused this memory to surface when he committed an act of telepathic memory invasion against Riker. (TNG: "Violations")