HELSINKI (Reuters) - Police said on Tuesday they had arrested a teenager suspected of aiding a shy chemistry student in making a home-made bomb that exploded in a crowded Finnish shopping mall last week, killing seven and injuring dozens.

They said the bomb had similarities to the home-made explosive used in the devastating 1995 Oklahoma City bombing, which killed 168.

The National Bureau of Investigation (NBI) said they had released three Finns late on Tuesday but kept under lock a 17-year-old Internet chat room friend of the prime suspect, 19-year-old Petri Gerdt who died in the blast.

"We have released three people because we don't have enough evidence to keep them in detention. But one person is still under arrest, a 17-year-old, because he is believed to have committed the crime of serious sabotage," NBI Deputy Chief Jari Liukku told Reuters.

Police in Finland can hold suspects for several days without charge.

The high school youth is believed, among other things, to have given advice on bomb-making over the Internet to Gerdt. Liukku declined to elaborate.

The home-made explosive, weighing up to 6.6 pounds and packed with metal shards, went off during peak shopping hours on Friday evening in one of Finland's largest malls in Vantaa, the quiet Helsinki middle-class suburb where Gerdt lived.

Police did not have a clear picture of his motive but said they did not believe the attack was linked to terror groups.

"The motive is the big question mark but we don't think it has an international connection or with international terrorism," NBI Inspector Rabbe von Hertzen told Reuters.

Tuesday was declared a day of mourning in the relatively crime-free country, which was stunned by the bombing. Flags were flown at half-mast and some official buildings were closed. Parliament held a minute of silence for what is the country's worst peacetime bomb attack.

DO-IT-YOURSELF BOMB CHAT SITE

Liukku said all four Finns, including the three just released from detention, had frequented a Finnish Internet chat room to exchange ideas with Gerdt on bomb-making, explosives and their effects.

The Finnish chat room "Forum for home chemistry" has since been shut down by authorities. Its host, who called himself Einstein, told national broadcaster Finnish YLE that the site was not used to plan an attack and he denied any involvement.

Police have not been able to pin-point how the bomb was made but said the explosive cocktail contained ammonium nitrate, a fertilizer used in the Oklahoma City bombing.

"It has similarities but I don't think he took the idea from Oklahoma," von Hertzen told Reuters.

Rolf Strandberg at Forcit Oy, a Finnish firm making explosives for the military, said: "It's quite easy to make a bomb if you can get ammonium nitrate. These different nitrates are available."

Police found evidence at Gerdt's home, including a computer and bomb-making information, that linked him to the crime and pointed them to chat rooms and other Internet sites.

Police said surveillance cameras and witnesses saw Gerdt arriving alone at the mall 20 minutes before the bomb exploded. They said he visited the mall, just a few hundred yards from his college, the day before as well.

Gerdt used the alias RC in the Finnish chat room but gave no clear clues to a planned bomb attack.

"I have not taken part in any larger accidents, but once I dreamt that a police car drove to the site of the explosion, but luckily I was already floating to other places," RC wrote on the Web site a few days before the blast.

RC also quoted lyrics from the late U.S. rapper Tupac Shakur, saying: "I ain't a killer but don't push me. Revenge is like the sweetest thing."