Isla Vista gunman Elliot Rodger convinced deputies that he was not a threat during a welfare check to his home last month, according to the local sheriff.
Family members contacted medical professionals expressing concern about the 22-year-old in April, but a check by police officials apparently left them convinced "that it was all a misunderstanding".
Santa Barbara County Sheriff Bill Brown told NBC News that Rodger was "able to convince them that he was not a threat to himself or to anyone else at the time".
He said the county's mental health and law enforcement services hoped to learn from the failure to prevent the tragedy.
More top news
A summary of the day's election campaign stories in under one minute.
Amanda Holden has told ITV News of the "horrendous" moment she feared her sister had been caught up in a deadly avalanche on Mount Everest.
Kim Kardashian has said she is "really happy" her step-father Bruce Jenner has found "inner peace" as he transitions to become a woman.