An explosion rattled a plant in the U.S. state of Oklahoma Sunday morning, igniting a fire but causing no injuries, local media reported. The incident happened at a manufacturing plant in Sweetwater, the website of NewsOK.com reported, without mentioning the name of the plant and what it manufactures. More than two feet of oil spilled out in the explosion. Firemen waited for that oil to burn away and then extinguished the fire. No one was injured in the incident. An investigation was under way. Authorities believed the blast was caused by a frozen valve, which caused pressure to build in a tank at the plant. The tank exploded, sending debris flying into nearby electrical wires, which ignited a fire.