|The news agency TT received an email warning with a threat to Sweden and its people ahead of the explosions [Reuters]
Swedish police have announced that they are treating bomb blasts in Stockholm, the capital, as a "terrorist crime".
Two people received minor injuries in the explosions on Saturday, the first of which came from a car that blew up near Drottninggatan, a busy shopping street in the centre of the city, Ulf Goransson, a spokesman for the Swedish police, told Al Jazeera.
Shortly afterwards, a second explosion was heard further up the same street and the body of a man was found on the ground.
"There was series of minor explosions, causing a fire in one of the cars in the street. Some minutes later, we found a man seriously injured 300 metres away from the scene of the first explosion. This man died," Goransson said.
Media in Sweden have named a 28-year-old man of Iraqi origin, who they say died in the attack, as the suspected bomber but Swedish police have refused to confirm the details.
"We are opening an investigation into a terrorist crime under Swedish laws," Anders Thornberg, of the National Security Service, told a news conference on Sunday.
Thornberg told the AFP news agency that his investigators were trying to find out if the blasts were part of a broader threat to Sweden or an isolated attack.
"We are trying to find out if something similar is going on [elsewhere in Sweden]. We don't have any indication about that, but we will try to make sure this was a single action," he said.
But he said it was too early to say if the blasts, about 15 minutes of each other, were linked. "We think so but we are still investigating," he said.
Linda Nyborg, a freelance journalist in Stockholm, said there was information saying that Britain's MI6 security agencies had information on the suspected perpetrator of the attack.
"The family has made comments on his death even though his name hasn't been made public or confirmed by police," she said.
"There is also information about a second man who was seen next to presumed suicide bomber. This is not official police information. So far police don't want to make any comment on the investigation."
Fredrik Reinfeldt, Sweden's prime minister, said it was too early to reach any conclusions and warned that speculation could lead to tensions.
"Sweden is an open society which has expressed a desire that people of different backgrounds, who believe in different gods or or no god at all, should be able to live side by side," he said.
"Our democracy is well-functioning. Anyone who feels frustration or anger has the possibility to express this in a non-violent way ... This is a society worth defending."
About ten minutes before the blasts, the Swedish news agency TT and the country's security services received an email warning with a threat to Sweden and its people ahead of the explosions.
It said the threat was linked to Sweden's presence in Afghanistan, where it has a force of 500 soldiers, and referred to caricatures of the Prophet Mohammed by Lars Vilks, a Swedish artist.
"Our acts will speak for themselves," TT quoted the message as saying. "Now your children, your daughters and your sisters will die as our brothers, our sisters and our children are dying."
The message concluded by urging "mujahidin" to rise up in Sweden and in Europe, the news agency said.
TT said the email was not sent anonymously but the news agency refused to say who the sender was.
The newspaper Aftonbladet cited sources saying the sender was a 28-year-old who had been expressing "extreme views" on the social networking website Facebook.
Police would not comment on a report in the paper which quoted a source as saying the man was carrying six pipebombs, of which one exploded, and a rucksack full of nails and suspected explosive material.
Vilks, who depicted the Prophet Mohammed with the body of a dog in a cartoon in 2007, has been the target of previous attacks.