"It looks like the man was alone, but we know it often involved more people. The explosions were quite well planned," Lindstrand said.

Swedish Chief Prosecutor Tomas Lindstrand (R) and Security Police official Anders Thornberg (L) attend a news conference in Stockholm, Sweden, Dec. 13, 2010. [Wu Ping/Xinhua]

He said the suicide bomber probably wanted to go to a special place, such as the crowded Ahlens shopping center or center of the city, where there were many people.

Lindstrand also confirmed the email to the Swedish Security Service (SAPO) and the Swedish News Agency TT was sent from the man's mobile phone or a broadband device.

Anders Thornberg, director of operations from SAPO, said they were going to contact Muslim organizations in Sweden.

"We stressed that it is about an individual, we do what we can to get out information about the case," Thornberg said.

Thornberg said his colleagues were working around the clock to process tips from the public as well as information from interviews and other sources.

He added that there was no reason to worry that another attack on Swedish soil was imminent, and they would not raise the threat level.

Two explosions took place in downtown Stockholm on Saturday afternoon. The attacker was killed by bombs he carried in his body in the second explosion.

Terrorism analysts say the mission seemed to be not correctly carried out. Otherwise, the two explosions could have killed or injured 10 to 100 people while currently only two were injured and the man himself was killed.

The man did not reach the crowd yet when one of the bombs exploded. The bomb belt was made up of 12 gas canisters, which officials believed was attached to Abdulwahab's stomach.