Firefighters on Tuesday extinguished the last of 20 burning tanks in an oil depot blaze they had been tackling for three days, as residents turned to the massive task of rebuilding.

Roy Wilshire, chief fire officer for Hertfordshire county, said there were still some small fires burning in small concrete containers, "but the tanks are out."

The fire started before dawn Sunday with a series of explosions at the Buncefield depot 25 miles north of London that blew the doors off houses in the surrounding area and spewed thick smoke as far away as France.

Authorities believe the explosions were an accident. Police said witness accounts and closed-circuit TV footage would be key to determining the cause, and they were trying to locate pieces of the shattered tanks to reconstruct the first one to explode.

The explosions came four days after an Al Qaeda videotape appeared on the Internet calling for attacks on facilities carrying oil, but officials drew no link.

"There is nothing to suggest that there is any malicious involvement in this," said Hertfordshire Police assistant chief constable Simon Parr.

Gemma Shepherdson, a spokeswoman for Total U.K., which operates Buncefield, said security was tight at the depot, the fifth largest in Britain. She said the site was ringed by video cameras, and people have to enter through a security gate and show ID.

Deputy Prime Minister John Prescott told Parliament there would be a full investigation once the fire was extinguished.

Meanwhile, a team from the British Meteorological Office flew through the smoke cloud Monday to gather data on its composition. Clare Lee, who led the team, said it was soot — "nothing more nasty than you'd get from a regular bonfire."

Dr. Jane Halpin, director of public health at the Hertfordshire and Bedfordshire Strategic Health Authority, told reporters Tuesday that air samples taken a day earlier "were very reassuring." Halpin said tests for asbestos in the smoke were negative.

The terminal stores about 4.2 million gallons of fuel. The fire has caused jitters on the global oil market and raised questions about safety at Britain's oil facilities.

Representatives of oil companies, pipeline operators and the airline industry met Monday to discuss ways of keeping fuel supplies flowing. French oil firm Total S.A. said it had implemented contingency plans to reroute supplies that normally run through the plant.

The Buncefield plant, which is part-owned by Texaco, carries jet fuel to Heathrow and Gatwick airports. It also stores gasoline, diesel and kerosene.

Forty-three people were treated for cuts and bruises from flying debris.

Scores of people had been evacuated from homes near the depot Sunday, and many were still waiting to return days later. Jane Hogg, mayor of Dacorum Borough Council, said authorities were trying to get people back in their homes soon, but first they needed to be sure the buildings were structurally sound.