Between five and 15 people are thought to have been killed by a huge explosion at a US fertiliser plant that witnesses said was "like a tornado".
More than 160 people were injured as dozens of homes and buildings were destroyed in the evening blast near Waco in the state of Texas.
Emergency services are still going from home to home trying to find survivors.
Three or four firefighters who had been tackling an earlier blaze at the site are still missing, police say.
Another member of the emergency services is now in hospital with severe injuries.
'Toll could rise'
The explosion happened at West Fertilizer on the edge of West, a town of about 2,700 people some 20 miles (32km) north of Waco.
The operation remains in "search-and-rescue mode" and is not yet in "recovery mode", Sgt William Swanton of the Waco police department told a news conference on Thursday.
Earlier, he said he could not give an accurate figure of the number of people killed in the explosion - but estimated between five and 15.
Other officials suggested the final death toll could be significantly higher.
Sgt Swanton said at least 160 people had been treated at various hospitals.
He said the blast site was being treated as a crime scene, but that this was procedural and he had "no indications this was anything other than an accidental fire".
He also said there had been a "small amount" of looting of properties in the town.
Emergency services officials said ammonia may have caused the explosion.
It has been reported the company had 54,000lbs (20 tonnes) of anhydrous ammonia on site.
In 2006, the plant was cited by the Texas Commission on Environmental Quality for failing to obtain or to qualify for a permit.
Police earlier said the half the town had been evacuated, amid fears of possible further explosions or a leak of toxic gas - though at an earlier press conference on Thursday morning, Sgt Swanton said he did not believe there was a threat from air quality.
The West Fertilizer plant is right on the edge of town, only a few hundred metres from houses, a school and nursing home.
Several witnesses described rushing to the nursing home to help residents, only to find it flattened - though it appears many people had already been evacuated because the fire was recognised as an explosion risk.
'Like a war zone'
TV images showed streams of emergency vehicles descending on the site and ambulance crews using a nearby sports field as an emergency treatment area. The injured were being carried to hospital not just in ambulances but in police squad cars and helicopters.
Glenn A Robinson, chief executive of Hillcrest Baptist Medical Center in Waco, told CNN the hospital was seeing "everything from orthopaedic injuries to patients that are experiencing serious blood loss".
McLennan County Sheriff Parnell McNamara said: "It's a lot of devastation. I've never seen anything like this. It looks like a war zone with all the debris."
US President Barack Obama said his administration was in close contact with emergency services at the scene.
"West is a town that many Texans hold near and dear to their hearts, and as residents continue to respond to this tragedy, they will have the support of the American people," he said in a statement.
Witness Debby Marak told the Associated Press news agency that she had seen smoke coming from the area near the plant and had driven over to see what was happening.
She said that when she arrived, two boys ran towards her screaming that the authorities had told them to leave because the fertiliser plant was going to explode.
She said she drove a short distance before the blast happened.
"It was like being in a tornado," she said. "Stuff was flying everywhere. It blew out my windshield. It was like the whole earth shook."
Another resident told KWTX-TV that she heard several explosions from 13 miles (20km) away.
"It sounded like three bombs going off very close to us,'' said Lydia Zimmerman.
The Dallas Morning News reported that West Fertilizer had told the Environmental Protection Agency that it presented no risk of fire or explosion.
The newspaper said it had seen documents in which the plant said it stored large amounts of anhydrous ammonia, but the worst scenario envisaged was a release of ammonia gas that would harm no-one.
An expert on fertilisers at the University of London, Prof Andrea Sella, told the BBC it was possible that the highly explosive ammonium nitrate had also been stored on site.
"Anhydrous ammonia will burn but it is unusual for it to detonate, whereas ammonium nitrate has a track record of blowing up," he told the BBC World Service.
"What we do know is that if you formulate it in the right way it actually forms kind of the bedrock of the mining industry - it's a hugely used commercial explosive."
The blast came almost on the anniversary of another ammonia explosion disaster in Texas.
On 16 April, 1947, a fire on a ship loaded with ammonium nitrate set off a series of explosions in the port of Texas City, and fires that burned for days. More than 500 people were killed and more than 1,000 buildings damaged in what remains the deadliest industrial accident in US history.