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
Michael Buble has been criticised as "disgusting" for posting a picture of a woman wearing tight shorts using the hashtag #beautifulbum.
Vinyl copies of a cover of Monty Python's 'Galaxy Song' recorded by Stephen Hawking go on sale later as part of Record Store Day.
The five-piece dance group do not sing but their choreographed back-flips had the crowd cheering at their audition in tonight's episode.