On 22 May, after the assassination of West Belfast Unionist MP William Twaddell, 350 IRA men were arrested in Belfast, crippling its organisation there. The largest single clash came in June, when British troops used artillery to dislodge an IRA unit from the village of Pettigo, killing seven, wounding six and taking four prisoners. This was the last major confrontation between the IRA and British forces in the period 1919–1922. The cycle of sectarian atrocities against civilians however continued into June 1922. May saw 75 people killed in Belfast and another 30 died there in June. Several thousand Catholics fled the violence and sought refuge in Glasgow and Dublin. On 17 June, in revenge for the killing of two Catholics by the B-Specials, Frank Aiken's IRA unit shot ten Protestant civilians, killing six in and around Altnaveigh, south Armagh. Three Special Constables were also killed in the shootings.