The National Bureau of Investigation (NBI) said the motive for the devastating explosion at a busy shopping centre on Friday evening, which injured around 80 people and shocked this peaceful Nordic nation, remained a mystery.
"He was likely a skilled bomb maker," NBI Deputy Chief Jari Liukku said. He said the explosives used were unusual and different from those used by the military.
Finnish media said the dead student may have obtained bomb-making information on the Internet, and said his parents, unaware of his activities and deeply shocked, were receiving counselling.
The shopping mall, the country's second biggest, in the Helsinki suburb of Vantaa, was packed with more than 2,000 people at the time.
The bomb, containing up to three kg (6.6 pounds) of explosives and metal shards, was detonated in the centre of the mall near a crowd of children watching a clown, police said. A seven-year-old child died and many of the injured lost limbs.
Police said the death toll would have been much higher had the bomb exploded a few minutes earlier, as a performance for children nearby had ended shortly before the blast.
The suspect was "an ordinary young man from a middle-class family. There was nothing particular about him or his personality," Liukku said.
Police declined to name the suspect as the investigation was still under way. Liukku said Finnish law prevents police from commenting in detail on a suspect's mental state.
Police searched the home of the student, who lived with his parents, and found material there that could be used to make a bomb. "In the home search, material was found which can connect the perpetrator's residence and the site of the incident," NBI Chief Tero Haapala told a news conference.
Finnish media said police had found Internet addresses and other information on the suspect's home computer that suggested he had obtained bomb-making instructions over the Internet.