An elementary school in the U.S. state of Oklahoma was briefly locke down Monday after a teacher found a loaded gun in his chair, local media reported. Police in the Oklahoma city of Moore said the teacher found the handgun in his chair after he arrived at Plaza Towers Elementary School for work Monday morning, according to local TV channel NewsOn6. Classes resumed at the school after a brief lockdown to allow for police to search the building to make sure there were no other weapons. There was a substitute teacher in the classroom Friday, but police believed that the substitute teacher did not bring the handgun to the school, said Jeremy Lewis with the Moore Police Department. Police were searching through surveillance video to find out how the gun ended up in the classroom, Lewis was quoted as saying.