An ammonia leak a the Storm Lake Tyson foods plant caused some momentary concern late Tuesday morning. The fire and police departments blocked streets around the plant and personnel from the plant were evacuated. Plans were made to evacuate a nearby elementary school and daycare center, but about 20 minutes later, public safety personnel were told by Tyson plant officials the leak had been shut off and the air was clearing. The leak was shut off before any students were moved away from the school or daycare center. An investigation at the plant determined the leak was caused by a manual control valve that had been improperly closed. A Tyson employee was taken to the Storm Lake hospital for exposure to the leaking gas.