The prisoners were then taken to a prepared site, shot in groups of ten to twenty and buried in a mass grave. The mass grave was discovered in October 1992 and guarded by the United Nations Protection Force which had deployed to the area earlier that year. In 1996, 200 sets of remains were exhumed from the grave by International Criminal Tribunal for the former Yugoslavia (ICTY) investigators. Croatia believes 61 others were buried in a different grave on the site, while ICTY prosecutors believe that figure stands at 60. The ICTY convicted two JNA officers in connection with the massacre, and also tried former Serbian President Slobodan Milošević for a number of war crimes, including those committed at Vukovar. Milošević died in prison before his trial could be completed. Several former members of the Croatian Serb TO and Serbian paramilitary units have been tried by the Serbian judiciary and convicted for their involvement in the massacre. In February 2015, the International Court of Justice ruled that the siege, massacre and simultaneous atrocities committed elsewhere in Croatia did not constitute genocide. The site of the mass grave is marked by a monument and the storage building used at Ovčara farm to hold the prisoners in captivity before their execution was rebuilt as a memorial centre in 2006. By July 2014, the centre had been visited by about 500,000 tourists. In 1990, ethnic tensions between Serbs and Croats worsened after the electoral defeat of the government of the Socialist Republic of Croatia by the Croatian Democratic Union ( – HDZ). The Yugoslav People's Army ( – JNA) confiscated Croatia's Territorial Defence ( – TO) weapons to minimize resistance. On 17 August, the tensions escalated into an open revolt of the Croatian Serbs. The rebellion was centred on the predominantly Serb-populated areas of the Dalmatian hinterland around Knin, approximately north-east of Split, as well as parts of Lika, Kordun, Banovina and eastern Slavonia. In January 1991, Serbia, supported by Montenegro and Serbia's provinces of Vojvodina and Kosovo, unsuccessfully tried to obtain the Yugoslav Presidency's approval for a JNA operation to disarm Croatia's security forces. The request was denied, and a bloodless skirmish between Serb insurgents and the Croatian special police occurred that March. This prompted the JNA itself to ask the Federal Presidency to grant it wartime authority and declare a state of emergency. Even though the request was backed by Serbia and its allies, on 15 March, the Federal Presidency declined.