Authorities say Norris Nguyen, who is diagnosed with schizophrenia, poses a serious threat to children.
According to investigators, the 24-year-old's specific target is the children at the Garden Grove elementary school he used to attend.
"We have evidence, we have statements, and we have the firm belief that he is committed, unfortunately, to harming children at this specific school," said Garden Grove Police Chief Kevin Raney.
Police say Nguyen escaped from the Royale Heath Care Center in Santa Ana around 4:45 p.m., where he has been committed since December 2011.
"Mr. Nguyen was allowed to exit the facility under the supervision of staff in order to exercise a little bit and get some fresh air, and he took advantage of that opportunity and he fled the location," Raney said.
Authorities say Nguyen's threats of violence escalated dramatically after the deadly shooting at Sandy Hook Elementary. The chief says Nguyen has written a lengthy and descriptive manifesto.
In August 2011, Nguyen was arrested after carrying an unloaded automatic weapon into the Garden Grove Police Station, telling officers he wanted to commit suicide by cop.
Authorities say Nguyen is Vietnamese, 5 feet 8 inches tall, weighing around 150 pounds with black hair and brown eyes.
Even though Nguyen has targeted a specific school and a specific teacher, officials fear the danger is even greater.
"He has certainly a historical fixation on this one school, but again, in my opinion, I think that every school and child in Orange County conceivably could be at risk," Raney said.
Authorities are not identifying the specific school, but say the district, school, staff and parents have all been made aware of the situation. Patrols have also been beefed up in the area. A statewide alert has also been issued.
If you see Nguyen, you are urged to call 911 immediately.