At least seven people remained hospitalized Friday from injuries they suffered from an ammonia leak in a small Oklahoma town as authorities focused on how the potentially deadly gas began spewing out of the tanker truck carrying it.

The leak Wednesday night from a truck outside a hotel in Weatherford forced at least 500 to 600 people to evacuate their homes early Thursday while others were ordered to remain inside theirs for several hours. Firefighters went door-to-door to tell those who needed to leave.

The truck was carrying 25,000 pounds (11,340 kilograms) of ammonia, and the U.S. Environmental Protection Agency has blamed a leaky gasket for the release of the gas. The company transporting the gas, Philadelphia-area-based supplier Airgas, said the leak had been contained.

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