A gunman shot and wounded two children aged 5 and 6 at a California school on Wednesday before shooting himself dead in what investigators said appeared to be an attack directed at the school's affiliation with the Seventh-Day Adventist Church.

The two wounded boys were in extremely critical condition and being treated at a trauma center, Butte County Sheriff Kory Honea told a press conference.

"I'm thankful that they are still alive but they have a long road ahead of them," Honea said.

The shooting occurred shortly after 1 p.m. (2100 GMT) at the Feather River School of Seventh-Day Adventists in a rural community near Oroville, California, about 55 miles (89 km) north of Sacramento.

The gunman had scheduled a meeting with a school administrator to discuss enrolling a student in what was a seemingly cordial encounter, Honea said.

But immediately after the meeting the shooter opened fire on students outside a classroom, then turned the handgun on himself, Honea said.

A California Highway Patrol officer who responded to an emergency call about an active shooter found the gunman dead at the scene with the handgun near his body. Officials transferred the remaining 35 students to a church where they were reunited with their families.

"We have received some information that leads us to believe that the subject responsible for the shooting targeted this school because of its affiliation with the Seventh-Day Adventist Church," Honea said, without elaborating.

The church, a Christian denomination, holds its observance on Saturday and adheres to a kosher Jewish diet, advocating vegetarianism.

The Feather River School serves children from kindergarten to the eighth grade, roughly from ages 5 to 13, according to its website.

Local officials sent a statewide alert to warn other schools affiliated with the church, and officers were sent to Seventh-Day Adventist schools in the nearby cities of Chico, Oroville and Paradise, Honea said.