An 860-student Catholic school was evacuated after some children became sick during a church service Wednesday, authorities said. At least two were taken to the hospital.

St. Hugo of the Hills Catholic School shut down classes shortly after 9 a.m. when at least three students passed out, said Bloomfield Hills Public Safety Director Rick Matott. Two of the children were taken to local hospitals, but their conditions were not believed to be serious, Matott told WJBK-TV.

Emergency medical crews and hazardous materials teams were sent to the school about 15 miles northwest of Detroit. Officials worked to determine what caused the children to become ill.

Bloomfield Hills fire crews could not find any indication that carbon monoxide was the cause, Matott said.

Students and staff walked across the street to Oakland Community College where parents were directed to pick up their children.