Branimir Donchev Delchev was born on 4 October 1957 in Sofia, Bulgaria, alongside his twin brother Delyan Delchev. They were raised in a wealthy household. Their father, Doncho Delchev, served as the Deputy General Director of DSO "Stara Planina", a position that kept him frequently abroad. His mother suffered from a severe mental illness either cyclophenia or schizophrenia and died by suicide in 1971 when Branimir was 14. She hanged herself in the attic of their family home.

Branimir had one older sister, Totka. Both of his siblings were also assessed for psychiatric issues after the shooting, and his brother was later diagnosed with an unspecified mental illness. Delyan would go on to die by suicide in 1988 after jumping from a balcony inside a mental hospital, following years of institutionalization.

Branimir reportedly showed signs of mental instability throughout adolescence. He was later diagnosed with severe schizophrenic psychosis, and was said to have inherited his condition from his mother. Despite this, he was a student at Sofia University at age 17, indicating high academic performance and social functionality before the shooting.

Privately, Branimir developed an obsessive attachment to his cousin-in-law, Tsvetelina, who had been adopted by his aunt and was not biologically related to him. She was a first-year chemistry student from a rural background. After she rejected his romantic advances, his mental state deteriorated further, culminating in a violent psychotic episode.

Branimir idolized cinematic portrayals of power and violence, particularly films like The Godfather. He was described as emotionally volatile, socially anxious, and intensely jealous. His diary entries revealed obsessive thoughts toward Tsvetelina and paranoid fears about political conspiracies.

On 25 December 1974, Branimir spent the evening watching The Godfather in a Sofia cinema with two friends, Valerie and Bozhidar. The movie, filled with themes of violence and control, reportedly had a disturbing effect on him. After the film, Branimir and Valerie returned to his home, where they found Delyan (his twin brother) and Tsvetelina present.

A heated argument broke out, likely fueled by jealousy over Tsvetelina. It escalated into a physical fight. Branimir, physically stronger than Delyan, overpowered him until Tsvetelina intervened in tears and left the house. She returned to her dormitory at Student City Dormitory Block №1.

Shortly after, Branimir retrieved his father’s .380 Walther PP pistol, several 7-round magazines, and a Finnish knife. He reportedly stopped at Valerie’s home and handled the pistol there before setting off for the dormitory, exhibiting erratic behavior, switching buses and trams repeatedly.

Branimir arrived at Dormitory Block №1 shortly before 8:00 p.m. He initially went to Room 410, Tsvetelina’s residence, but she wasn’t there. Sources vary on whether he fired inside the room or simply left after finding it empty.

The mass shooting began in Room 519, where several students were celebrating a Vietnamese exchange student’s birthday. Branimir opened fire, killing three people and injuring others. He then moved through the hallways, shooting and stabbing people at random. When the halls cleared, he knocked on doors, firing into rooms sometimes killing those inside, including a young couple expecting a child in Room 315.

Some of the victims were foreign nationals, including students from Vietnam and Latin America. He used both his pistol and knife, wounding multiple students in the process.

At one point, two students Imet North and Stoyan Agov attempted to subdue him. He seriously injured both; Agov died later in hospital. Branimir was eventually restrained by student Atanas Nachev, who tackled and held him in a locked room. During this, Branimir reportedly screamed that he was a “special agent” on a secret mission, and falsely claimed there was a second shooter.

The shooting lasted an estimated 15 to 20 minutes. Six victims died on site, and two more died from injuries in hospital. Eight others were wounded. The shooting remains the deadliest mass shooting in Bulgarian history.