|LONDON, England (CNN) -- British authorities say they have contained a fire raging at a fuel depot that injured 43 people, but it will take a day to burn itself out.
The blast followed a series of explosions at the facility about 40 kilometers (25 miles) north of London early Sunday, lighting up the predawn sky and sparking a massive blaze.
Britain has been on edge for terrorist attacks since four suicide bombers killed 52 commuters in July on London's transport system, but a police statement ruled out any attack.
"All indications are, at this stage, that this was an accident," Hertfordshire Chief Constable Frank Whiteley said.
"However, clearly we will keep an open mind ... until we can confirm that for certain."
Forty-three people were taken to hospitals for their injuries, Whitely said, but the majority were "walking casualties." All but two have been released, he said, and those two do not have life-threatening injuries.
Although firefighting units with more than 150 firefighters were on the scene, authorities said the fire may have to burn itself out.
"Given the latest info on the fire, the fire is likely to continue for the next 24 hours or so," Whitely said.
"It could go into days," Roy Wilsher, fire department spokesman, told reporters.
He described the fire as "the largest I've seen."
Wilsher said 250 million liters (more than 66 million gallons) of foam concentrate -- to be mixed with water to create flame suppressant foam -- was "on the way to us."
"We're working with the oil industry experts to see if even that's enough," he said.
About 20 petroleum tanks were thought to be involved in the blaze at the Buncefield Oil Depot near Hemel Hempstead, said Jane Vine, spokeswoman for Hertfordshire Fire and Rescue.
"What we're really concerned about is toxic smoke from the area," Vine told CNN.
"There's a heavy cloud. We don't know if it's toxic, but we're asking people in Hemel Hempstead to keep their windows and doors closed and stay inside."
The smoke was drifting slowly east, Whiteley said.
Residents closest to the depot were being evacuated, she said, and nearby roads, including the M1 and M10, were closed. "What we really want is for people to keep away from the area."
Nearby residents left quickly, some throwing possessions in the backs of their cars. Whiteley said those with doors and windows damaged by the blasts should leave.
Whitely said more explosions were possible, but they would not be as powerful as the first blasts.
Hemel Hempstead residents reported hearing a loud boom about 6 a.m. local time. Two other blasts followed, British television network ITN reported, at 6:26 and 6:27 a.m.
The network showed flames shooting an estimated 80 to 100 meters (up to 300 feet) in the air, along with an immense cloud of smoke blocking out the sun. Witnesses told ITN the blasts shattered windows and caused other damage at nearby homes.
As the smoke spread across Hemel Hempstead, witnesses said there was a strange taste in the air.
"It's a horrible feeling in the back of your throat," Andy Dicker told the network. "Very unpleasant."
At least one of the blasts was enough to wake residents of west London, some 40 miles away.
Video from the scene showed heavy damage to buildings at the depot and nearby roads covered in debris from the blasts.
Witness Simon Heyward described to ITN seeing "what looked like fireworks shooting up into the sky." Police, he said, had cordoned off the area.
Another woman said she thought the noise was an earthquake or a plane crash at the Luton Airport. "It was that kind of intense noise," she said.
Texaco owns 40 percent of Buncefield, but does not operate the facility. The other 60 percent is owned by Total, according to Texaco-Chevron UK and U.S.-based Texaco.
No American workers were at the site, a Texaco USA spokesman said. Although the terminal is the fifth-largest in Britain, Whiteley told reporters there should be no concern about a fuel shortage.