A fire broke out again at the Buncefield oil depot overnight when petrol vapour reignited in a tank previously undamaged by the blaze.
The fire service said the flames were safely contained and would be allowed to burn themselves out.
Firefighters have worked day and night to extinguish the three-day inferno in Hemel Hempstead, Herts, but health experts warned that the risk to public health was not over.
Thick ground-level soot which could cause people breathing difficulties is expected to build up over the next few days, as the dense plume of smoke and toxins which has spread across the south-east of England falls.
There were angry calls for a public inquiry into the explosions that started the blaze, and into why the emergency services were unpractised for such a large fire. Financial experts said that insurance claims from homeowners and businesses whose properties were destroyed in the first explosions - which measured 2.4 on the Richter scale, equivalent to a small earthquake - could reach "hundreds of millions" of pounds.
The Fire Brigades Union said that Hertfordshire Fire Authority had been "woefully prepared to deal with all but the most minor oil fire".
The county's chief fire officer, Roy Wilsher said his officers had been "magnificent" but admitted that they had never prepared to tackle such an "apocalyptic" blaze. "All previous experience had been on one tank [of oil]," he said. "I think with any incident of this kind there will undoubtedly be some lessons to be learned."
The 58-hour operation to extinguish fires in 20 oil tanks, which began shortly after 6am on Sunday morning and had to be stopped several times for fear that personnel would be killed by exploding drums, ended at 4.45pm yesterday. With the smothering of the largest tank on site, the " infamous tank 12", 650 firefighters from 17 services across the country had deployed 15m litres of water and a quarter of a million litres of foam concentrate. A small number will remain at the scene for several days, cooling the area to ensure that the tanks do not reignite.
Several spoke of fear as they arrived on Sunday morning at what would become the largest peacetime blaze in Europe. "It was a complete fireball. It does not compare to anything I have ever seen," said Jon Smith, station commander at Hemel Hempstead.
Inside the terminal, which used to store 16 million litres of fuel, a number of small fires away from the tanks burned on. Crushed cars lay beside the scorched shells of abandoned offices ripped apart in the blasts.
Several businessmen were able to return to the nearby industrial estate to inspect damage to their premises. Some residents evacuated from the exclusion zone around Buncefield were allowed to return with police escorts. They were advised to wipe up dust or soot left by the fire, throw away unwrapped food and re-wash clothes left out on a line. Insurance claims could run into the "hundreds of millions" of pounds, predicted Geoff Miller, an analyst with Bridgewell Securities in London.
Conservative Hemel Hempstead MP Mike Penning called for a public inquiry into the fire and questioned why town planners had allowed homes to be built progressively closer to the depot since its construction in the 1960s.
The Health and Safety Executive will hold a full investigation, the findings of which will be made public. Fire services minister Jim Fitzpatrick said it was too early to say whether or not there would be a public inquiry into the fire.
Health experts tried to reassure the public that there would be no long-term health repercussions. Professor Warren Lenney, of the British Lung Foundation, said "nasty toxic substances" created by the smoke could cause short-term irritation and coughing, particularly to asthma sufferers and the elderly. But he said that few people had been exposed to high-density smoke and thought it unlikely that cancer-causing toxins such as carbon monoxide would be an issue.
Air samples taken on Monday were "very reassuring", according to Dr Jane Halpin, Hertfordshire and Bedfordshire's director of public health. Tests for asbestos in the smoke were "clear and negative".
About 200 schools within a 10-mile radius of Buncefield remained closed yesterday because of possible dangers to children from smoke particles. However, all but those closest to the depot were expected to reopen today. Residents nearby were still advised to stay indoors where possible and keep their windows shut.
French oil firm Total SA, which operates the depot, repeated denials that there had been leaks in the run-up to the explosions - considered by chemical engineers to be the likeliest cause.
The impact of Europe's worst peacetime blaze
What risk do the fumes pose to public health?
There are concerns that soot could cause problems at ground level. Experts say the problem is short-term airway irritation caused by soot particles. Tests for asbestos were clear and the Met Office said the smoke was nothing "more nasty than you'd get from a regular bonfire".
What effect will it have on the environment?
The main threat came from the toxic sludge of chemicals created when foam and water were sprayed on burning fuel, which might have poisoned local water supplies. Firefighters say that concrete bunds (containers) around oil tanks have prevented this. Agricultural experts will analyse the potential fall-out on local vegetables and livestock. Council officials have said that foam landing around Hemel Hempstead is not toxic.
How much will the blast cost?
Insurance claims from the fire could run to "hundreds of millions" of pounds, according to securities analysts.
Will this affect UK energy supplies?
Downing Street acknowledges there could be "short-term local difficulties" but has urged drivers not to panic-buy. Oil companies say there are four larger depots in the UK and the country had a refining over-capacity before the blast. The main challenge for suppliers is one of logistics: they now need to transport oil further.
What caused the fire?
The first blast was probably an accident, police say, but we may never know exactly what happened. Chemical engineers believe the explosion was probably a huge leak of gasoline. Any electrical spark could then have detonated the concentrated cloud of vapour. Total SA, which operates Buncefield, denies there were leaks in the run-up to the explosions. Police and the Government have dismissed terrorism as the cause.
What happens now?
A Health and Safety Executive team will reconstruct shattered oil tanks to determine the cause of the blast. Police have interviewed six people whop were on-site at the time and are viewing CCTV footage. The ferocity of the blaze will have destroyed evidence.
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