Meanwhile, the IRA actions in Belfast, such as the killing of policemen, resulted in more retaliation attacks directly on the Roman Catholic population by loyalists, sometimes covertly aided by state forces. Atrocities and assassinations were perpetrated by both sides across Belfast. The McMahon Murders of 26 March 1922, and the Arnon Street Massacre of a week later, in which uniformed police shot a total of twelve Catholic civilians dead in reprisal for the killings of policemen, were two of the worst incidents. On 22 May, the IRA assassinated unionist politician William Twaddell, in Belfast. Immediately afterwards, the Special Powers Act was passed in an effort to stop the chaos. Internment (arrest and imprisonment without trial) was introduced and over 350 IRA men were arrested in Belfast, crippling its organisation there. In June 1922, after the Treaty was approved by public referendum, the Republican movement in the South split into two warring factions; the Pro-Treaty and Anti-Treaty forces, marking the beginning of the Irish Civil War that would last almost a year and result in several war crimes and travesties between southern Irishmen. In the North, the cycle of sectarian atrocities against civilians in Belfast continued into Summer 1922. May saw seventy-five people killed in Belfast and another 30 died there in June. Several thousand Catholics fled the violence and sought refuge in Glasgow and Dublin. However, after this crisis, the violence declined rapidly. Only six people lost their lives in July and August and the final conflict-related killing took place in October 1922.