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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ITV News correspondent Dan Rivers was taken, by regime minders, to the hospital where those who were affected by the attack were treated.
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The composer says schools are failing to see how music can support behavioural and academic success.