The violence intensified in the early 1970s, with rival paramilitary groups being formed on both sides. Bombing, assassination and street violence formed a backdrop to life throughout The Troubles. The Ulster Volunteer Force killed 15 people in the McGurk's Bar bombing in 1971. The PIRA detonated twenty-two bombs, all in a confined area in the city centre in 1972, on what is known as "Bloody Friday", killing nine people. Loyalist paramilitaries, the Ulster Volunteer Force (UVF) and Ulster Defence Association (UDA) retaliated against the PIRA campaign by killing Catholics at random from around August 1972 onward. A particularly notorious group, based on the Shankill Road in the mid 1970s became known as the Shankill Butchers.