Killed because he had a different skin colour... a watershed in Norway

Norwegian PM Jens Stoltenberg

Police say three other neo-Nazis arrested after the attack have been released, although they may still face charges. A sixth is being sought.

All are linked to a group known as the Boot Boys and were arrested in an Oslo flat filed with Nazi memorabilia.

"Everything implies... Benjamin Hermansen was killed because he had a different skin colour," Prime Minister Jens Stoltenberg told a news conference on Monday night.

"If this is the case, it marks a watershed in Norway.

"We have not experienced something like this before and we must do all we can to avoid it happening again."

Voice against racism

Benjamin Hermansen - the son of a Norwegian woman and a Ghanaian man - was well known as a campaigner against racism.

Police scour the murder scene for evidence

Hundreds of people attended a rally on Sunday at the scene of his killing.

The Norwegian Government says it will now step up its efforts to clamp down on racially-motivated crime, although it has played down suggestions of a ban on neo-Nazi groups.

Police believe there are probably about 150 active hardcore neo-Nazis in Norway's population of 4.4 million.