Stockholm, Sweden (CNN) -- A bomber who apparently killed himself in central Stockholm on Saturday was probably on his way to a more crowded location, but his bomb went off prematurely, Swedish authorities said Monday.
He sent a warning to a Swedish news agency from his phone shortly before the explosion, chief prosecutor Thomas Lindstrand said in the first official confirmation of a link between the blast and the warning.
The warning came from a man called Taimour Abdulwahab, the news agency that got the e-mail told CNN Monday.
Lindstrand said authorities were almost certain that he was the bomber, but they have not yet carried out DNA testing or spoken to his family.
A car exploded shortly before Abdulwahab's bomb, injuring two people; police are also linking that blast to him.
No one other than Abdulwahab was killed.
The Swedish Security Service said the incidents appeared to be the work of "a single perpetrator."
He appears to have been acting alone at the time of the explosion, Lindstrand said Monday.
Police believe that he had some kind of outside help in preparing the explosives, but they have no additional suspects at the moment, Lindstrand said.
On Sunday, Anders Thornberg of the Swedish Security Police called the explosions "an act of terrorism."
But he said Monday that there was no reason for the public to be worried and that Sweden was not raising its terror threat level.
The two explosions occurred within minutes of each other Saturday in a district full of Christmas shoppers, Swedish authorities said. Swedish news agency TT and police said they received e-mailed threats 10 minutes before the explosions.
U.S. federal agents are en route to Stockholm and are expected to assist Swedish authorities with the investigation, Security Police spokeswoman Anna-Maria Bok said.
"They are sending a team of seven bomb experts who will assist us," she said. "They are experts at tracing explosives, and they have knowledge of what explosives that have been used at other attacks in the world."
FBI spokesman Richard Kolko said "a small team of experts" was sent after Swedish authorities requested U.S. assistance in investigating the bombings.
Prime Minister Fredrik Reinfeldt called the explosions "completely unacceptable" as he mounted an impassioned defense of Swedish society in the face of what seems to be its first suicide bombing.
"This is not the path we want to go down," he said Sunday.
"Sweden is an open society," Reinfeldt added. "It is an open society which has demonstrated a will that people must be able to come from different backgrounds, believe in different gods or not believe in any god at all. Be able to live side by side, together, in our open society."
UK Prime Minister David Cameron phoned his Swedish counterpart Sunday to express his support, Reinfeldt press secretary Roberta Alenius said.
"He called him because they are close friends," Alenius said. "They expressed their mutual involvement in the fight against terrorism, and they also stressed that the two countries' authorities will continue to cooperate closely in this investigation."
Regional police chief Carin Gotblad said the bomber had "failed."
"While this was a very serious event, no innocent people were seriously hurt. If that was the intention, then the perpetrator failed," she said.
On Sunday night, London's Metropolitan Police searched a property in Bedfordshire, north of London, in connection with the Stockholm investigation, a spokesman said.
The spokesman, who would not give his name, said that the warrant was executed under Britain's Terrorism Act 2000 and that "no arrests have been made ... and no hazardous substances have been found."
The writer of the e-mailed threats to the Swedish news agency and police mentioned the presence of Swedish troops in Afghanistan and a Swedish cartoonist who depicted the prophet Mohammed, according to TT, a Swedish news wire that received the threats.
The e-mails contained sound files featuring a person speaking in Swedish and Arabic, TT reported.
About 500 Swedish troops are in Afghanistan, according to NATO's International Security Assistance Force.
The sender referred to Swedish silence regarding the Afghanistan troops and the controversial cartoon by Lars Vilks that depicted Mohammed as having the body of a dog.
"Now your children, daughters and sisters will die like our brothers and sisters and children are dying," the e-mail states, according to TT.
"Our actions will speak for themselves," the person said in an audio recording. "As long as you don't end your war against Islam and the humiliation against the prophet and with your stupid support to Lars Vilks the pig."
Thornberg said over the weekend that authorities are working on both intelligence and a normal police investigation and will try to determine "if there may be any more acts of terrorism like this one being planned. At this point, we don't think there are more acts coming, but we can't say for sure."
Qadeer Baksh, chairman of the Luton Islamic Centre, a mosque north of London, said he had a "series of confrontations" with Abdulwahab during the month of Ramadan in either 2006 or 2007.
He said Abdulwahab was propagating an extremist message around the mosque, "sowing the seeds of violent radicalization" by preaching violent jihad as an obligation for every Muslim.
Baksh said Abdulwahab also accused other Muslim leaders of apostasy, denouncing them as untrustworthy and claimed they were working for the government.
The two men parted ways after a final confrontation, and Abdulwahab did not return to the mosque, Baksh said.
CNN's Andrew Carey contributed to this report.