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.
More top news
The UN has said it is deeply concerned by reports hundreds of Syrian men have disappeared amid a mass exodus from rebel-held areas.
A man suspected of planning a terrorist attack has been arrested in Rotterdam after police found a loaded AK-47 in his apartment.
Another mild day with highs reaching up to 13C degrees in mild southerly winds