Students at Freeman High School in the tiny town of Rockford, south of Spokane, say they knew about the videos.
A freshman who witnessed the shooting told local KREM-TV that the shooter, a classmate since elementary school, stalked the hallway with a pistol and second gun, appearing calm as he fired at his victims and the ceiling.
He also said the shooter was obsessed with other school shootings.
The three other victims who were shot are reportedly in stable condition.
"Today that student came to school, armed".
Spokane Public Schools said on Twitter Wednesday morning that all schools in the district were "in full lockdown as a precautionary measure", and later tweeted the lockdown had been lifted. "Today he chose to bring guns to school including a rifle which was stored in his large duffel bag and a handgun inside his coat pocket".
The name of the shooter has not been released, but he is reported to be a sophomore.