Deputy head of the Swiss federal intelligence agency (NDB) Jürg Bühler told the NZZ am Sonntag newspaper that the agency had been trawling through online forums since the Oslo attacks.
"We haven't found anyone who actually approved of the massacre," he told the paper on Sunday. "But there are people who praised the ideology of Breivik's manifesto. And this ideology certainly has a tendency to violence."
Bühler said they were currently talking about "less than ten people," but that this number could grow as investigations continue. He said the NDB was currently liaising with canton authorities to find out more about the subjects.
"We are finding out whether the subjects have already come to the attention of other authorities," said Bühler. "Then we will try to evaluate their potential for violence and find out if they are part of a scene."
Bühler added, "If it becomes clear that they only sympathize with Breivik on ideological grounds, the law does not allow us to continue our investigations – only if there is an actual threat of violence."
Preventative surveillance is a controversial topic in Switzerland – the parliament voted down a bill widening police powers two years ago, though the government is reportedly preparing a new bill for next year.
"The time could be ripe for further surveillance measures," said Bühler.