'..but they should not have been just wandering around.'

Not sure what you would have people do, round up everybody in the country, just in case?

There had been an explosion in Oslo, 50 km away. It was completely unclear what had happened, surely you have seen the confusion in the news reports during the first few hours. By all accounts people on the island had gathered in groups, discussing the news of the explosion. Who could have foreseen the man dressed as a policeman was going to gun them down. It was the most unlikeliest of scenarios. One cannot be prepared for something like that.