A lawyer of film director Peter Rodger says the family called police several weeks ago after being alarmed by YouTube videos posted by his son "regarding suicide and the killing of people".
The attorney said police had conducted an investigation and interviewed the man. Police said the man appeared to have trouble making friends but did not find a history of guns.
Police have not formally named the suspect, who died following the attack.
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Armed police were out in force as runners competed in the largest public event in the city since Monday's terror attack.
Taliesin Myrddin Namkai Meche, 23, was stabbed along with Rick Best, 53, when they tried to stop a racist attack on two Muslim girls.
The shadow home secretary, 63, said she worked in the Home Office for nearly three years when she was a trainee.