The 19-year-old Finnish chemistry student Petri Erkki Tapio Gerdt who killed himself and six others by setting off a bomb inside a mall Oct. 11 frequently visited a Web site for builders of home-made explosives, Finnish media reported.
The Web site also contained a chat room, the press said.
"Police are investigating an Internet forum for chemistry-interested people where the participants seem to know both the perpetrator and how to make bombs," the daily Hufvudstadsbladet reported.
The leading Swedish-language daily quotes Tero Haapala, in charge of the investigation into the bombing, as saying "it is under investigation, and this lead is one" that police are currently investigating.
Haapala was not immediately available for comment.
A day earlier, Haapala would not confirm that the bomber had found instructions on how to build a bomb on the Internet, saying only that analysis of "confiscated IT material" would begin this week.
The home-made bomb contained up to 2 kilograms of explosive material and a large number of lead and steel shotgun pellets, apparently aimed at killing as many people as possible, police said.
Almost 100 people were injured in the attack, with dozens needing emergency surgery, officials said.
Investigators did not say whether the home-made explosive device was of the pipe bomb type, which typically contains these kinds of materials.
"We know it was not military explosives, but we do not yet know the bomb's explosive material," Kimmo Himberg, a forensic expert, said.
The explosion occurred Oct.11 evening on the ground floor of an atrium of the Myyrmanni shopping mall, located in the Myyrmaeki suburb in the city of Vantaa, 12 kilometers (seven miles) north of the capital Helsinki.
At the time, up to 2,000 people were estimated to be in the mall, one of Finland's largest, doing their shopping ahead of the weekend, officials said.
A number of victims remained hospitalized Oct.14, but none was in a life-threatening condition.