A shooting spree Friday at a youth camp outside the Norwegian capital, Oslo, could have claimed at least 10 lives, police told daily Dadgbladet.
The information from the island of Utoya was however sketchy, police spokesman Bjorn Erik Sem-Jackobsen told the newspaper, dpa reported.
Public broadcaster NRK meanwhile quoted a witness as saying the shooting could have claimed 25 to 30 lives.
Police said they had arrested a man suspected of the rampage at the camp. The man was according to some witnesses wearing a police uniform.
Norwegian online media published some images of the island's shores with pixellated photos of what could be bodies on the shoreline of the island of Utoya where the camp was held.
Meanwhile, a group that called itself "Helpers of the global jihad" was reported to have claimed it conducted the bombing in Oslo, according to the New York Times and the Swedish daily Expressen.