The lawyer for Hollywood filmmaker Peter Rodger said that the "family believes the child was the perpetrator".
Alan Shifman said the son had Asperger's syndrome, had trouble making friends and had been receiving professional help.
Mr Shifman added that the family had contacted police about "recent posts on YouTube" by Elliot "regarding suicide and the killing of people". The family is "devastated" and co-operating with police, he said.
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There will be blustery showers across northern parts overnight with showery rain and sleet across the south.