Rescue teams are searching the rubble of homes for survivors after a massive explosion ripped through a nearby fertiliser plant in Texas.
Up to 15 people are feared to have been killed and more than 160 others were injured in the blast in the small town of West, near Waco.
The Dallas Fire-Rescue department has confirmed one of its firefighters, 52-year-old Captain Kenny Harris, died as he helped fight the initial blaze.
Police have warned of a "very volatile situation" because of ammonium nitrate found at the plant. The chemical compound is commonly used a fertiliser but can also be used in explosives.
Hundreds of worshippers prayed for the victims at a church service on Thursday.
"We know everyone that was there first, in the beginning," said Christina Rodarte, 46, who has lived in West for 27 years.
"There's no words for it. It is a small community, and everyone knows the first responders, because anytime there's anything going on, the fire department is right there, all volunteer."
Firefighters had been tackling a blaze at the factory for about 20 minutes when the explosion rocked the area on Wednesday evening.
Three to four volunteer firefighters are among the missing, said Sergeant William Patrick Swanton.
Capt Harris, who lived in West, was off duty when the fire broke out but decided to help the town's volunteer crew. He was described as a married father of three grown sons.
The blast destroyed as many as 80 houses and reduced a complex of 50 apartments to a "skeleton standing up", according to one official.
It also struck a nearby school and nursing home, where an evacuation saw 133 patients taken out of the building, some in wheelchairs.
The explosion at the West Fertiliser Company could be heard 45 miles away. It sent flames shooting into the night sky and rained burning embers, shrapnel and debris down on shocked and frightened residents.
Sgt Swanton said the number of dead was estimated to be between five and 15, and emergency crews were moving from house to house in a search and rescue operation.
More than half of the 2,700 population of West had been evacuated and people were still being pulled from damaged buildings.
The explosion had a magnitude of 2.1 - equivalent to a minor earthquake, according to the US Geological Survey.
A man filmed the initial fire, and captured the moment of the explosion on camera.
His child is heard shouting: "Dad, I can't hear, let's get out of here. I can't hear anything." The dad says simply: "Oh my god."
Waco Assistant Fire Chief Don Yeager said it was an anhydrous (without water) ammonia explosion.
Anhydrous ammonia is a nitrogen-hydrogen gas widely used as a fertiliser, but it is also a key component of many explosive devices.
Residents in West have been urged to remain indoors because of the threat of new explosions or leaks of ammonia from the plant's ruins.
The town's mayor Tommy Muska, who is part of the team of local volunteer firefighters, compared the blast to a nuclear bomb going off.
In a statement, President Barack Obama offered the prayers of the nation to the people of West.
"A tight-knit community has been shaken, and good, hard-working people have lost their lives," he said.
Texas Governor Rick Perry described "a nightmare scenario for that community," as he announced he was seeking a federal disaster declaration which would make additional funds available.
Debby Marak said she noticed a lot of smoke coming from the area across town near the plant.
She said she drove over to see what was happening, and when she got out of her car two boys ran towards her screaming that officials told them to leave because the plant was going to explode.
Moments later the blast happened.
"It was like being in a tornado," the 58-year-old said. "Stuff was flying everywhere. It blew out my windshield. It was like the whole earth shook."
The cause of the fire remains unknown and it is being treated as a crime scene, which is standard procedure, Sgt Swanton said.
But he added there was no indication the blast was anything other than an industrial accident.
The explosion came on the eve of the 20th anniversary of the Waco siege - a deadly confrontation between federal authorities and heavily armed locals.
The Dallas Morning News said that the fertiliser company previously reported to the US Environmental Protection Agency and local public safety officials that there was no risk of fire or explosion at the plant.