A Finnish teenager set off a bomb this week in a crowded shopping mall near Helsinki, killing himself and six other people in Finland’s worst incidence of violence since World War II. Some 80 people were injured, many seriously, from shards of metal packed into the 5-pound bomb. Police did not release the name of the presumed perpetrator, a chemistry student at Helsinki University. They said they found explosives in the boy’s home and bomb-making instructions, downloaded from the Internet, on his computer—but no clue as to the motive. “We have had some accidents in Finland,” said Interior Minister Ville Itaelae, “but nothing like this has ever happened before.”