9 May 1991 is accepted as her official date of death. That evening she left her family's countryside home outside Novosibirsk and did not return. Her body was found by a fisherman on 17 May in the Inya River. She was presumed to have drowned near Novorodnikovo Train Station and been carried 40 kilometers by the current. On 19 May, she was buried in Novosibirsk's Zayeltsovskoye Cemetery.
The exact time, place, and conditions surrounding Dyagileva's death are unknown. Some believe that she committed suicide, others that she drowned accidentally. Her death was ruled an accident, based on the forensic evidence. Proponents of the theory that she committed suicide point to the fact that her step-brother and close friend, Sergei Shurakov, had just died of causes related to medical malpractice, on 23 April 1991, affecting her deeply. There were also rumors that Dyagileva was murdered, as it was alleged that her skull had been fractured and there was no water in her lungs (indicating that she died before falling in the river). There was no formal investigation into this allegation. Unconfirmed accounts indicate that a few of Dyagileva's close friends received postcards from her on 10 May 1991. The text was (approximately): "May everything be good with you. I love you very much. May God protect you from misfortune."