Police who visited a British-born gunman three weeks before he killed six students in California were aware he had posted disturbing videos online but did not watch them.
A statement from Santa Barbara County sheriff's department corrected earlier claims that officers were unaware of any video when they checked on Elliot Rodger on April 30.
The statement also provided new details on the sequence of events during a visit to Rodger's apartment at a time when he was plotting the rampage, in which he apparently took his own life.
The guns he used in the killings were stashed inside his apartment at the time, but police never searched the residence or conducted a check to determine if he owned firearms because they did not consider him a threat.
The statement does not explain why the videos were not viewed or whether the officers knew anything about the contents beyond a description of them being "disturbing".
The sheriff's department also revealed new details about the timeline leading up to the killings. It said Rodger uploaded his final video to YouTube detailing his "Day of Retribution" and stating his plans and reasons for the killings, at 9.17pm on May 23, the day of the shootings.
One minute later, he emailed a lengthy written manifesto to his mother, father and therapist that also detailed his plans and contempt for everyone he felt were responsible for his sexual frustrations and overall miserable existence.
The first gunshots were reported at 9.27pm. The rampage was over and Rodger was dead just eight minutes later.
Police injured Rodger during two separate shootouts at a community near the University of California, Santa Barbara, leaving a trail of bloodshed that ended with Rodger apparently shooting himself in the head before crashing his black BMW into a parked car.
Thirteen people were injured, eight from gunshot wounds, four from being hit by his car and one who suffered a minor injury.
The timing indicates that Rodger stabbed to death three people in the apartment some time earlier.
According to the statement from the sheriff's office, four deputies, a police officer and a dispatcher in training were sent to Rodger's apartment after being informed by the county's mental health hotline that Rodger's therapist and mother were concerned about videos he posted online.
The visit lasted about 10 minutes, during which officers found him shy and polite. The deputies questioned him about the videos. Rodger told them he was having trouble fitting in socially and the videos were "merely a way of expressing himself".
Like many other states, California has a law intended to identify and confine dangerously unstable people before they can do harm. It allows authorities to hold people in a mental hospital for up to 72 hours for observation.