With less than two years to go until the opening of the London Olympics, the venues are nearing completion, the project remains on budget and the city is gearing up to welcome the world in 2012.

Beyond the tangible progress, however, lies a lingering concern: Will London be safe from a terrorist attack?

London has been a recent target of international terrorism: The day after the capital was awarded the Olympics, four homegrown suicide bombers attacked the city's subway and bus network, killing 52 commuters and themselves in July 2005.

Investigators believe the group were likely guided by al-Qaida operatives.

The head of Britain's transport police at that time of the 2005 attacks was Sir Ian Johnston, and he's director of security for the London Olympic organizing committee.

Johnston says the official threat level during the games will be set at "severe" — just one notch below the most extreme level of "critical."

"You can't run at critical for very long," Johnston said during a three-day world press briefing on Olympic preparations this week. "We can escalate to critical, but when you're running at severe, that's already a pretty high level of threat."

London's threat level was last classified as "critical" in July 2007, and has been set as "severe" — meaning a terrorist attack is highly likely — since January.

Johnston said organizers will operate a "100-percent search regime," meaning all spectators, officials and media will be checked and their bags scanned or searched upon entry at Olympic venues. People will be prohibited from taking liquids, even bottled water, into the sites.

"I think people will understand that it is for their safety," Johnston said, referring to a measure that is a throwback to the tight global airline restrictions imposed after the foiled 2006 plot to blow up trans-Atlantic planes with liquid explosives.

The tight spectator security in London marks a shift from last February's Winter Olympics in Vancouver, where checks were limited and often random. Johnston said the threat level in Vancouver was extremely low.

Johnston — who has had a 44-year career in policing, including eight years as chief constable of British Transport Police — said London will avoid being heavy-handed and seek to run "effective and discreet" security.

Security has been a top concern at the Olympics since the slaying of 11 Israeli athletes and coaches at the 1972 Munich Games. Olympic planners have ramped up security following the Sept. 11, 2001, attacks in the United States.

Britain is viewed as a high security threat, and the Olympics offer a platform for any terrorist group looking for maximum impact and publicity.

"If you rank the order of the countries that al-Qaida wants to do things to, it's the 'Great Satan' (United States) first, and it's the Brits second," said Bob Ayers, a London-based former U.S. intelligence officer. "Here's this massive event coming up. You know exactly when it's going to occur ... If you're a terrorist planner, it doesn't get any better than this."

Ayers said stopping a determined suicide bomber would be "impossible," stressing that the key work will be done in the leadup to the games as security officials monitor communications and try to break up any plots in the planning stages.

The task of guarding the Olympics is huge: Protecting 10,500 athletes, 15,000 officials, 25,000 media and hundreds of thousands of visitors and spectators.

About 12,000 police officers will be on duty each day — more than double the 5,000 per day used to guard a Group of 20 summit meeting last year in east London, close to the Olympic site.

The centerpiece complex is the 1-square-mile Olympic Park in the Stratford area, where venues include the 80,000-seat main stadium, aquatics center, velodrome, basketball and handball arenas and the main media center. The outer structures of the venues are mostly complete.

Up to 250,000 spectators a day are expected to flood into the park during the games.

But the Olympics also feature venues around the city, including Lord's cricket ground (archery), Wimbledon (tennis), Horse Guard's Parade (beach volleyball) and the O2 Arena (basketball and gymnastics). The sailing venue is in Weymouth on England's south coast, and soccer matches will be played across Britain, including Glasgow, Manchester, Cardiff and Coventry. National Olympic teams will also have pre-games training camps around the country.

Security outside the Olympic venues is another major challenge, particularly on the public transport network. Hundreds of thousands of people will be riding subway trains and the Javelin shuttle train running from central London to the Olympic Park.

Johnston said it is impossible to screen every passenger and that police sniffer dogs may be put to use.

The International Olympic Committee has expressed full confidence in Britain's security plans.

"The one issue no city can guarantee is security," senior Australian IOC member Kevan Gosper said. "That varies from country to country and city to city. We know London has the best people in place. This a country that has been affected by terrorism and is not new to security issues. They are about as good as it would get anywhere in the world."

Britain's new Conservative-led coalition government is urgently reviewing security plans for the Olympics.

It has pledged not to cut the $950 million core security budget, but there will likely be pressure not to use a $380 million contingency fund. Britain's government is imposing major cuts across all government departments as it attempts to erase the country's national debt.

John Patten, a member of the House of Lords and former government minister, has accused the previous Labour-led government of failing to address security issues away from the centerpiece venues.

"While the high level of security at the Olympic Park is welcome and evident, the other venues are ... wide open to severe security risk," Patten told a recent House of Lords debate. "This is something to which those with responsibility in this area must turn."

Patten said in June that the most vulnerable Olympic sites may be venues outside central London and training facilities. He said authorities are also worried about the potential use of improvised chemical bombs — known as ICDs — and cyber attacks to disable venue and transport ticketing systems.

Gen. David Richards, the incoming head of Britain's armed forces, indicated the military may provide specialist teams — usually deployed in Afghanistan — to check for roadside bombs.

Defense Secretary Liam Fox confirmed last month that discussions are under way to determine the role Britain's military will have in protecting the games.

Britain's spy agencies and police will share information on threats through a dedicated Olympic intelligence center.

London will also likely see a range of new technologies deployed to protect the Olympics, including CCTV systems to check for suspicious activity among spectators and unmanned drone aircraft to monitor crowds.

"Ultimately there are limits to how much you can do to make things secure, and at the same time allow a city to carry on going about its business," said Stephen Graham, an academic and author of "Cities Under Siege: The New Military Urbanism."


Associated Press Writer David Stringer contributed to this report.