The man in charge of investigating the massive fires at a Hertfordshire oil depot on Sunday says the flames may have destroyed all clues to the cause.
Deputy Prime Minister John Prescott has visited the scene of the blasts which injured 43 people, two seriously.
The fire chief described the incident at the Buncefield fuel depot near Hemel Hempstead, after 0600 GMT, as possibly the largest in peacetime Europe.
The M1 finally reopened around 2200 GMT after being closed for 12 hours.
Hertfordshire's Chief Fire Officer Roy Wilsher said: "The damage a fire of this intensity will cause may, or may not, leave clues for the fire investigation team."
Junction 8 of the motorway, aswell as some slip roads, however, remain closed. The Highways Agency said it would review the situation on Monday morning.
The fire, which police believe was an accident, could burn for another day.
About 2,000 people living near the site have been evacuated, while police have advised others to keep their windows and doors closed because of fumes.
By nightfall a police cordon remained around the site.
Thick clouds of smoke are continuing to spread to the south-east and south-west of the site.
One person admitted to Watford General Hospital in intensive care with respiratory problems has now been stabilised. Another person is in Hemel Hempstead Hospital being kept under observation.
The other 41 people were treated for minor injuries and discharged.
Witnesses said another two explosions followed the first at 0626 GMT and 0627 GMT at the site near junction 8.
A security worker at the depot, Troy Woodland, described what happened.
"I sat down and all of a sudden there was a huge orange light and a massive explosion which blew the doors through and knocked me off my chair, and the ceiling fell in," he told the BBC.
In total, 20 petrol tanks were involved, each said to hold three million gallons of fuel.
A police investigation into the incident has begun, including investigations by anti-terrorist police.
But Chief Con Whiteley said there was "nothing to suggest anything other than an accident".
On his visit to the site Mr Prescott praised the response and offered government help but was told the emergency services were able to cope.
Hertfordshire's Chief Fire Officer Roy Wilsher said: "This is possibly the largest incident of its kind in peacetime Europe."
Fire chiefs are consulting oil industry experts about using millions of litres of foam to quell the blaze.
But Mr Wilsher said they needed to know they had enough foam before they began.
The BBC's Gavin Hewitt said about 100 firefighters were waiting to attack the blaze.
Meanwhile samples of smoke are being taken to determine the long term effects of exposure, if any, according to Dr Jane Halpin, director of Hertfordshire Public Health.
The blast was heard as far away as Holland
She said: "However, what I would restate is that those people who are most at risk are those people who have inhaled the smoke."
Tanker driver Paul Turner said he ran for his life after the explosion lifted him off his feet.
"I just saw this great big ball of fire come up from behind the building. It was about 50 metres wide," he told the BBC.
"Then there was the loudest explosion I have ever heard in my life. I got up, turned around and ran to my car and sped out of there as fast as I could."
Many houses have been damaged, with some reporting feeling effects from the explosion as far away as Oxfordshire - while it was heard in a number of counties and even France and the Netherlands.
Eye witnesses reported buckled front doors, cracked walls and blown-out windows.
Of the 2,000 people evacuated 290 people have gone to a leisure centre while 50 others have been offered bed and breakfast accommodation.
The M10 motorway is closed in both directions between junction 1 and junction 7 as well as some arterial roads in Hemel Hempstead.
Motorists have been told not to go "anywhere near the M1 from the M25 upwards".
Hertfordshire police said about 70 schools in the Hemel Hempstead and St Albans areas would also be closed on Monday.
At Heathrow airport some flights were forced to delay landing because of smoke, but Luton airport was operating as usual.
The Buncefield depot is a major distribution terminal operated by Total and part-owned by Texaco, storing oil, petrol as well as kerosene which supplies airports across the region, including Heathrow and Luton.
The country's fifth largest fuel distribution depot, it is also used by BP, Shell and British Pipeline.
Police said there was no indication the explosion would cause fuel shortages and warned against panic-buying.
A spokesman for Total said: "We are doing everything we can to support the emergency services and to bring the situation under control."
A spokesman for the Department for Trade and Industry said it was too early to say what the effect would be on fuel supply but oil companies were getting oil from other parts of the south east and across the UK.
A spokesman for the Health and Safety Executive said it would be investigating the incident.
Concerned relatives can call a police casualty bureau on 0800 096 0095, or from abroad on 0207 1580125.
BUNCEFIELD FUEL DEPOT BLASTS
With light winds blowing to the south-east and south-west, smoke is likely to be spread over large parts of southern England by Monday