"He travelled on the ferry boat from the mainland over to that little inland island posing as a police officer, saying he was there to do research in connection with the bomb blasts, " NRK journalist Ole Torp told the BBC.

BBC: Norway hit by deadly bomb and shooting attacks

He said research in Britain, Australia and the US suggested that a warning or a caution from a police officer tended to stop stalking but only "if it happens before the obsessive behaviour becomes ingrained".

BBC: When love becomes fatal obsession