At the time of the genocide, Seromba was priest of a Catholic parish at Nyange in the Kibuye province of western Rwanda. He was charged with the deaths of around 2,000 Tutsis who took refuge in his parish church. According to the charges brought against him, Seromba ordered his church to be bulldozed on April 6, 1994, and then shot some survivors.
Seromba fled Rwanda in July 1994. Catholic monks helped him move to Italy, change his name and also helped him work as a priest for the Catholic Church near the city of Florence using the alias Anastasio Sumba Bura. Under pressure from Carla Del Ponte, the then Chief UN War Crimes Prosecutor, Seromba surrendered himself to the International Criminal Tribunal for Rwanda (ICTR) on February 6, 2002. On February 8, 2002, he pleaded not guilty to the charges of genocide, complicity in genocide, conspiracy to commit genocide and extermination as a crime against humanity. His trial began on September 20, 2004, before the Third Trial Chamber of the ICTR. On 13 December 2006, he was found guilty and sentenced to 15 years in prison.
Seromba appealed the verdict. On 12 March 2008, the Appeals Chamber of the International Criminal Tribunal for Rwanda (ICTR) decided his responsibility was even greater than previously found, affirmed his conviction, and increased his punishment to life in prison.
On 27 June 2009, Seromba was transferred to Benin. Seromba is serving his life sentence at Akpro-Missérété prison at Porto-Novo, Benin.