Elisabeth Breivik, who lives in Los Angeles, sent an email warning that the then-30-year old man was obsessed with computer games and rarely seemed to leave his mother's home.
Norwegian law authorities travelled to California this weekend to interview her for more details.
"She has been able to give us an important piece of the puzzle," Police lawyer Pål-Fredrik Hjort Kraby said on Sunday after interviewing Ms Breivik. "We don't want to give details of the interview, but we got a good result, which will be very useful in the court case."
Forensic psychiatrists Torgeir Husby and Synne Soerheim last week concluded that Mr Breivik was a paranoid schizophrenic who had been in a state of psychosis at the time that he shot 69 people, mostly teenagers, dead on the Island of Utøya, in July. If this is backed by an independent medical board, it means Mr Breivik is likely to face compulsory treatment rather than prison.
Although once close to his half-sister, Anders Breivik also seems to have seen her as an example of degraded modern humanity. Read on and comment » | Richard Orange, Malmö | Sunday, December 04, 2011
