Investigators in Texas say there is no evidence that terrorists attacked a BP oil plant after an explosion killed 15 and injured more than 100 on Wednesday.
Flames erupted from the plant after the blast
FBI agents have pulled out of BP's Texas City plant, site of a blast that shook buildings five miles (8km) away.
Sections of the plant were reduced to cinders and rubble in the explosion on Wednesday, which sparked a big fire.
The refinery, the third-biggest in the US, has had previous safety problems including another blast in March 2004.
That explosion forced an evacuation and cost the company $63,000 in fines.
Last September, two workers died and another was seriously injured when they were scalded by superheated water that escaped from a high-pressure pipe.
Wednesday's explosion happened at 1330 (1930 GMT) at the western end of the plant, which covers an area of almost 500 hectares.
BP said the blast happened in an isomerisation unit, used to produce octane for petrol. It had been working normally before the explosion, company officials said.
"It's clear that we have a lot of work to do in the coming days to make sure we understand exactly what happened, and we're going to do that," said BP American President Ross Pillari.
Employee Charles Gregory said he felt the floor rumbling as the blast erupted.
"It was real scary," he told the Associated Press news agency. "Have you ever heard the thunder real loud? It was like 10 times that."
Thick clouds of black smoke rose into the air as firefighters struggled to douse the flames.
The aftermath of the blast left a gaping hole in the earth, while chunks of charred metal littered the ground. Cars parked nearby were coated with a layer of ash.
One man who had remained unaccounted for was confirmed dead on Thursday afternoon, bringing the death toll to 15.
The Texas City refinery, 35 miles (55km) south-east of Houston, is the third biggest in the US, employing 2,000 people.
It processes roughly 450,000 barrels of crude oil each day, amounting to about 3% of domestic oil supply in the US and one-third of BP's output across the country.
BP spokesman Bill Stephens said investigators had started looking into the cause of the explosion.
"We're going to be very thorough on this one," he said. "I can't tell you exactly when we will finish this investigation, but no stone will go unturned."
Investigators are poring over the wreckage
Another company spokesman, Hugh Depland, virtually ruled out terrorism as a factor.
He was backed by an FBI spokesman, who said agents had pulled out of the plant on Thursday.
Health officials said more than 70 of those injured had been working inside the refinery and some of them were in a critical condition.
Texas City was the scene of the country's worst industrial accident in April 1947, when a ship full of ammonium nitrate, used in fertiliser, exploded in the harbour, killing at least 576 people.
Were you in the area when the explosion occurred?
This debate is now closed. Thank you for your comments
My wife and I were at home about 10 minutes drive from the refinery. We heard a loud noise like thunder. First we thought it might be from the housing construction site in our area. When she turned on the TV to the news channel, there it was on the Breaking News. She promptly called the hospital she works and was called to duty at the Emergency Room. It was very terrifying. I am yet to find what happened to our Church building which is directly opposite the refinery.
Bernice & Samuel Nkansah-Adjei, Texas City, TX, USA
A good friend of mine works there (she was not injured but was very, very frightened). This is a safety issue and NOT terrorists; why is every little ripple a terrorist act? However, these oil companies are not held accountable enough for unequivocally making certain employees and residents are protected.
Baddi Singh, The Woodlands, Texas, USA
I have family in Texas City, we have been in contact with them throughout the day. They told us there was a huge explosion that vibrated through the air and ground knocking the pictures off the walls and breaking some of the windows in their neighbourhood. They stated that they were initially advised to remain indoors but this was soon lifted and as most of the injured were people living in the town there is a sense of loss throughout the town.
Arthur Claxton, Elmwood USA/Ramsgate UK