Officers arrested a 15-year-old student who attends the school on Wednesday and say he confessed to posting the threatening messages and had acted alone. They say no weapons were found at his Ontario home.
They released the initial suspect who had been arrested Tuesday, identified as a former student who had been expelled by the school previously for unrelated reasons.
The online threats included tweets such as "I'm recreating Columbine" and "Chino needs a good shooting," according to a group known as "The Tactical Institute," who spotted the tweets and reached out to law enforcement.
"The Chino Police Department and Chino Valley Unified School District will continue to take all threats of violence seriously" Chino Police Chief Karen Comstock said. "School safety remains a top priority and we will use all resources necessary to ensure that the parents, students and school faculty are safe on campus."