A hazardous materials crew gave the all-clear after investigating an "unknown odor" at Fontana Middle School that sickened eight people, including one who was hospitalized, the San Bernardino County Fire Department said.
The noxious odor was reported by students and staff members about 7 a.m.
Seven people received medical attention and one other, an adult, was taken to the emergency room, firefighters said.
Crews searched for nearly three hours but were unable to find the source of the odor, which had faded.
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"The preliminary findings indicate that something external from the school is creating an odor during a short window of time in the early morning," the Fontana Unified School District said in a statement.
Classes remained in session and an investigation was launched.