Oklahoma tornado: What causes storms in 'Tornado Alley'?
Oklahoma is particularly vulnerable to tornadoes, lying inside the so-called 'Tornado Alley', stretching from South Dakota to Central Texas.
The geography and climate of the region frequently create the conditions for huge thunderstorms - with warm, wet air blowing in from the Gulf of Mexico meeting cold, dry air coming from the massive Rocky Mountain range, hemmed in by air masses on the eastern part of the country.
Three-quarters of the world's tornadoes occur in the United States - around 1000 per year - but the patterns of how and when they will strike are still difficult to predict.
David Shukman reports.