School and Wright City Fire Protection District officials said the evacuation was no more than a "precaution." "We did not believe there was any danger to the students at any time," said Fire Chief Brian Nickerson. "But we decided to err on the side of caution just to be completely safe." Officials said the smell of gas was detected at approximately 9:30 a.m. and appeared to be concentrated around some classrooms on the building's north side. Fire district officials were immediately called in and the building of all 429 students, teachers and building personnel was evacuated. Nickerson said no students were sick and none complained of headache or nausea from the gas fumes. "It was a very orderly, very well-run process," said district spokesperson Jill Smoot of the students evacuation. "We had the students out of the building quickly and without incident." Smoot said fifth- and sixth-grade students were taken to Wright City Elementary School, while seventh- and eighth-grade students were sent to Wright City High School. "We had activities set up for them and the students had their lunch as scheduled," Smoot added. Once the building was cleared, a crew from AmerenUE and one from the fire district went into the building to try to determine the origin of the leak. After about a half-hour search, the crews reported they could find no trace of where the leak might have originated. "We checked every room," said Nickerson. But shortly before 11 a.m., a valve on a rooftop mechanical air handling heating/cooling unit was identified as the source of the leak by members of the Ameren crew. The crew tested the valve and confirmed it was the origin point. The valve was shut off to prevent any further possible leakage. Ameren and fire district officials then conducted a second inspection throughout the building to ensure no gas fumes were present. School officials decided to allow students return to the building shortly after noon. "We sent out word to the parents through our school dispatch communication system," reported Smoot. A repair crew from TMI, a St. Charles-based mechanical equipment specialist, was called in to replace the valve, school officials said. Until the leak's source was positively identified, Nickerson speculated the gas fumes might have come from a nearby housing subdivision, rather than from inside the school itself. By 11 a.m., all firefighting crews from both Wright City and Warrenton and the Warren County Ambulance District had left the school property.