LONDON -- The flames are finally out at the enormous oil depot fire north of London, but they have set off a blaze of questions about safety at huge chemical and industrial plants around the country.

Experts are questioning the wisdom of allowing such potentially dangerous installations in densely populated areas. The country's five biggest oil depots are in or near major metropolitan areas, meaning an accident or terrorist attack could be catastrophic.

Some critics also worry that emergency services are insufficiently prepared for such disasters.

The series of explosions Sunday at Buncefield, 25 miles outside the capital, came four days after an al-Qaida videotape appeared on the Internet calling for attacks on facilities carrying oil. However, officials drew no link between the two events, and the blasts are being treated as an accident.

Though safety procedures at oil depots are generally rigorous, it is impossible to eliminate risk at facilities storing large amounts of volatile fuel.

The dangers posed by those facilities have increased in recent years, with suburban sprawl placing people ever closer to the plants. When it was built in 1968, the Buncefield depot was surrounded by farmland.

Buncefield is Britain's fifth-largest oil depot and a major hub in its fuel pipeline network. The depot stored 4.2 million gallons of gasoline, diesel, kerosene and aviation fuel.

Now, 80,000 people in the commuter town of Hemel Hempstead push up against its outer rim, with houses just a few hundred yards away. The industrial park is right beside it.

Deputy Prime Minister John Prescott's office, responsible for regional planning, said it was too soon to discuss whether the fire should prompt a reexamination of where such plants are placed. Three are close to London, one is near Liverpool and the other is near Glasgow, Scotland.

Critics also say emergency services have failed to keep up with the growing risks. Britain's Fire Brigades Union slammed the local department's response to the Buncefield blaze, saying firefighters lacked foam and specialized equipment they needed until supplies were brought in from elsewhere in the country.

"Quite simply, they didn't believe they were going to have a fire," union spokesman Duncan Milligan said.

The Hertfordshire Fire Authority defended its work, saying departments around the country keep some foam on hand and share when major blazes occur -- a system it said worked well.

Bob Woodward, who is leading the government investigation, said he had no "grave concerns" about Buncefield before the explosions, and he was confident the cause would be pinpointed.

Emily Ablett, a spokeswoman for Total U.K., the company that runs Buncefield, said it had excellent safety procedures in place and would cooperate with the probe.

Investigators will have to wait for the site to be declared safe before examining it, and they may find that the flames left little forensic evidence behind.

The government's Health and Safety Executive will have to begin with the basics: finding where explosive fumes leaked into the air and figuring out what ignited them.

Investigators will interview witnesses, examine closed circuit television footage and look at computer logs recording data about the tanks, including temperature, fuel levels and what kind of substances they store, said Phil Reed, a fire investigator in Scotland.

But some of the depot's neighbors, like Alice Wong, feared further danger.

"I'm worried about another explosion," she said.