Waxahachie, Texas (CNN) -- Preliminary air quality tests show "no threat to the public" from a massive fire at a Texas chemical plant Monday, city and federal officials said.





Waxahachie Fire Chief David Hudgins said authorities believe that the fire "overran the sprinkler system," and it sent thick plumes of smoke high into the sky hours after it started. There were no flames visible by late afternoon, though by then the blaze might have discharged dangerous substances into the air, according to the federal Environmental Protection Agency.





That prompted EPA staff to dispatch air monitors to test for toxic vapors. Around 4:45 p.m. (5:45 p.m. EST), Waxahachie city spokeswoman Amy Hollywood said initial tests did not indicate any danger to those in the area. That assessment was later confirmed by EPA official Nicolas Brescia.





"We have not seen any significant level that would cause a public health concern," Brescia told reporters.





Brescia said that tests conducted from flights over and around the fire detected small amounts of two chemicals, but nothing that would be considered dangerous to anyone outside the immediate vicinity of the fire. A statement from his federal agency also indicated that "ground-level portable sampling devices did not show elevated toxic chemicals."





Earlier, Dave Bary -- a Dallas-based spokesman for the EPA -- said the most prevalent chemical that the company had documented, and produced on site, was anhydrous ammonia.





The EPA official said earlier Monday that 48,630 pounds of the chemical, which creates a toxic vapor downwind when released, have been reported at the site. It was not known how much of the chemical was released into the air during the fire.





Officials ordered the evacuation of a school, an apartment building and other industrial sites within an eight-block radius of the plant, said Diana Buckley, an official with Ellis County government. A shelter-in-place advisory is in effect for some smaller nearby towns northwest of the city, including Red Oak, she said.





"Everybody is out and safe," Donald Golden, environmental health and safety manager for Magnablend Inc., the plant's owner, said of employees and visitors to the plant.





The plant stores and mixes numerous chemicals, said Amy Hollywood, a city spokeswoman.





Images transmitted by CNN affiliates KTVT and WFAA showed orange flames devouring buildings and licking at tanker cars parked at the edge of the Magnablend plant, which is about 30 miles south of Dallas.





Firefighters had pulled back and were using ladder trucks to spray water on the fire, Hollywood said.





Multiple fire departments were on the scene. One of the them, the Midlothian Fire Department, reported that chemicals had been released in the fire, but no additional details were immediately available.





Officials have said burning liquid continued to fuel the flames hours after it began, but they appeared able to wrestle the fire under control by early Monday evening.





There was no immediate word on the cause on the fire, Golden said. No one was injured in the fire, Hudgins said.





The nearby Wedgeworth Elementary School was evacuated, a school district official confirmed. Students were being taken to a school across town, school officials said.





Navarro College's Waxahachie campus also evacuated, a receptionist confirmed. Southwestern Assemblies of God University, which is on the other side of town from the chemical plant, was keeping people inside but had not evacuated, spokeswoman Christina Freeze said.





Magnablend produces custom chemicals for a variety of industries, including oil, agriculture, pet and animal feed, water treatment, construction and industrial cleaning companies, according to its website.





Anhydrous ammonia is a clear, colorless gas, according to the Centers for Disease Control and Prevention. It can be absorbed into the body by inhalation, ingestion, eye contact and skin contact.





CNN's Ed Lavandera and Dave Alsup contributed to this report.
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