Rigged Vehicle Rams Terminal At U.K. Airport

Chris Driver-Williams, a retired British army major who is a consultant to the U.S. and British governments on bombs manufactured by terrorist groups, said the two car bombs may have appeared crude but their explosive firepower could have been "catastrophic."

According to some reports, the valves on the gas cylinders were left open, which Driver-Williams said was a strong indication that the bombers intended to create a "fuel-air" explosion. "What you're effectively creating is a thermobaric device," he said, referring to an explosion that relies on oxygen in the atmosphere to ignite a blast wave more powerful than many conventional explosives.

Fuel-air explosives are difficult to ignite, and the timing is especially tricky, Driver-Williams said. A mistake might result in two separate but minor explosions -- such as a burning car -- if the fuel-air mixture does not ignite properly. If it does, however, the result would be a "fireball the size of a house."

Magnus Ranstorp, a Swedish terrorism expert who consults closely with British officials, said the threat in the United Kingdom is the gravest in Europe.

"Without luck and without the public's help, there's no way they will be able to stem the tide that is coming toward them," said Ranstorp, research director of the Center for Asymmetric Threat Studies at the Swedish National Defense College in Stockholm. "There is no question that the United Kingdom is still facing an onslaught of [terrorist] cells that are ready and willing to launch attacks like this."

Britain is a target, he said, in part because it is seen as the closest ally of the United States and its biggest military partner in Iraq.

Almost all terrorist plots carried out by Islamic extremists in the past five years in Britain have included investigative trails that led to Pakistan. With an estimated 1 million people of Pakistani descent living in Britain, and hundreds of thousands traveling each year between the two countries, it is particularly difficult to track those involved in plots hatched or guided by al-Qaeda forces based in Pakistan.

British officials have warned that the threat of terrorism has grown sharply in the past three years and have questioned whether British police and intelligence services are capable of fully investigating each suspicious individual or threat.

In November, shortly before she resigned as director of MI5, the national security agency, Eliza Manningham-Buller said 1,600 people were under surveillance, comprising 200 terrorist cells or groups. She also estimated that there were 30 active "plots to kill people and to damage our economy." British officials say the number of people under surveillance has grown since then.

"The problem is of such magnitude that it surpasses all other European countries in terms of the threat level," Ranstorp said.

Michael Chertoff, secretary of U.S. homeland security, said that "in an abundance of caution" during the Fourth of July holiday period, security measures would be increased at U.S. airports and on transportation routes.

U.S. federal security officials said there would be more police at airport curbs and more police dogs trained to sniff out explosives. British officials were also tightening security around the ongoing Wimbledon tennis tournament and Sunday's concert in memory of Princess Diana, which is expected to draw 60,000 to Wembley Stadium in London.