Up to 17,000 people were asked to evacuate homes near a chemical plant in North Carolina after a blast inside the compound that injured at least 18.
The ball of chemical flame was clearly visible over Apex
The explosion rocked the waste disposal plant near the town of Apex, sending chemical clouds high into the sky.
The cause of the blast was unclear, but officials said the fire it sparked might take days to extinguish.
State environmental officials said, however, that initial air quality tests had found nothing alarming.
Mayor Keith Weatherly said the flames appeared to have set four petrol tankers on fire overnight.
He also said that rain falling on the town on Friday morning was a good thing. "It scrubs the air," he said.
About half of Apex's 32,000 residents were told to evacuate, with more told to leave their homes hours later.
There was no immediate confirmation of what chemicals were contained in the gas cloud that spilled over Apex.
But local officials said initial suspicions suggested it may have been chlorine gas.
The plant, operated by EQ Industrial Services, handles a range of industrial waste products including paints and solvents, and contains chemicals including chlorine, pesticides, herbicides, sulphur and fertiliser.
The town of Apex is 10 miles (16km) south-west of Raleigh, the state capital.
Although evacuation orders were issued overnight, some disregarded instructions to leave their homes.
Roseanne Smith, of Raleigh, said her work colleague was ordered out of his home in the middle of the night.
"Schools are being used as shelters. People have been asked by the police not to come to the area, for fear of smoke inhalation," she said.
Officials warned that those who deliberately stayed near the site of the blast would not stay for long.
"People are going to want to come and sightsee at this fire scene," town manager Bruce Radford told the Associated Press.
"They will either get terribly sick or they will be arrested."
Residents staying at home were urged to close all windows, turn off their air conditioning units and look out for symptoms of chemical poisoning.
Some 25 people work at the EQ plant, although all were thought to have left by the time the fire broke out on Thursday night.
Those hurt included police officers and a firefighter, who were taken to hospital with respiratory complaints.
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