"Now the Islamic state has been created. We now exist here in Europe and in Sweden. We are a reality," the voice said in the file, submitted to The Associated Press by TT. "I don't want to say more about this. Our actions will speak for themselves."
Swedish Prime Minister Fredrik Reinfeldt said Sunday the attack was "unacceptable" but urged Swedes not to jump to "premature conclusions" that "create tension which paints pictures that are then difficult to change."
"Sweden is an open society ... which has stated a wish that people should be able to have different backgrounds, believe in different gods ... and live side by side in our open society," Reinfeldt said at a news conference.
Swedes, with a tradition of welcoming immigrants and a culture of transparency, began questioning the veracity of their self-image as a secure nation after the 1986 murder of Prime Minister Olof Palme. In 2003, the fatal stabbing of Foreign Minister Anna Lindh in an department store was another wake-up call.
But there have been no major terrorist strikes.
"We had a terrorist attack in the 1970s from the Rote Armee Friktion of Germany, but if this is a suicide bomber it is the first time in Sweden," security police spokesman Anders Thornberg told The Associated Press. "It's very serious and it's very tragic that these things have come to Sweden too."
On Sunday, the pedestrian district where the explosions occurred was eerily quiet and empty for a mid-December weekend.
"We're used to seeing things like this on the news. This was a lot closer to home but it still doesn't feel very tangible," said Eric Osterman, a 26-year-old student.