Complex sentence: On 22 July 2011, he set off a car bomb amid government buildings in Oslo, killing eight people, then shot dead 69 more people (mostly teenagers) at a Workers' Youth League (AUF) camp on the island of Utøya.
Split into simpler sentences: On 22 July 2011, he bombed government buildings in Oslo, killing eight people. He then killed 69 more people, mostly teenagers, in a mass shooting at a Workers' Youth League (AUF) camp on the island of Utøya.