Norwegian police said on Monday two men had been arrested on Sunday on suspicion of involvement in a mass shooting that killed two people and wounded 21 in and around an Oslo gay bar in June.

A Somali citizen in his 40s and a Norwegian one in his 30s, each with prior involvement in criminal activity, were arrested by police on Sunday, Oslo police said in a statement.

Only one suspect, named by police as Zaniar Matapour, was arrested at the scene of the June 25 shooting, while an international arrest warrant has been issued for a fourth suspect.

Police last week said its hypothesis that the shooting was an act of terrorism had been strengthened during its investigation.