The shooter was dressed as a police officer, and said he had come over for a routine check following the bombing event in Oslo. He signalled and asked people to gather around him before firing his weapons, killing and injuring many people. He first shot people on the island and later started shooting at people who were trying to escape by swimming across the lake. The police reported that most of the casualties were youths of about 15 and 16 years old. People on the island were reported to have hidden in lavatories or undergrowth, communicating by text message to avoid giving their positions away to the gunman.