A 22-year-old Norwegian man accused of killing his step-sister before opening fire in a mosque near Oslo in August was charged with murder and terrorism on Monday, prosecutors said.

Philip Manshaus was arrested after opening fire in the Al-Noor mosque in the affluent Oslo suburb of Baerum on August 10 last year before he was overpowered by a 65-year-old man.

Just three worshippers were in the mosque at the time, and there were no serious injuries. The body of his 17-year-old step-sister was later found in their home. Adopted by his father's girlfriend, Johanne Zhangjia Ihle-Hansen was killed by four bullets, police said.

Police have previously said they believed the motive for the murder to be racist, saying he killed her because she was of Asian origin. The charge sheet filed with the Asker and Baerum district court on Monday contained two charges. One charge of murder for having killed his stepsister, and one charge of a "terrorist act" by attempting to kill with the "intention of creating severe fear in a population."

The trial is expected to begin on May 7. Manshaus has previously admitted to the actions but has rejected the charges of murder and terrorism, claiming that it was a "kind of self-defence." On September 9, at a court hearing to extend his detention in custody, Manshaus raised his arm in a Nazi salute to the assembled media.

Manshaus lawyer, Unni Fries, told broadcaster NRK that the charges did not come as a surprise. "We are going to take a closer look at this and work towards the trial," Fried said.