It was morning rush hour on the London underground.
Dressed in jeans and with a backpack, Hasib Hussain, a visitor from northern England, slipped quietly into the crowd. But, as the world would soon discover, he was no ordinary commuter making his way across the bustling capital.
On July 7, 2005, Hussain, 18, and three other young suicide bombers detonated their explosives-filled backpacks on London's transport network, killing 52 people and injuring at least 700 others.
The single worst terrorist attack on British soil came within 24 hours of London winning the bid to host the 2012 Olympic Games. Suddenly, a jubilant nation was plunged into mourning.
With less than a month to go before the Games begin on July 27, reports emerged this week that two British Muslim converts had been arrested in London on terror charges.
Now, almost seven years on from the 7/7 bombings, experts have told The Straits Times about how the terror threat to London has evolved.
The Lone Wolf
While the likelihood of an atrocity on the scale of 7/7 has receded, there is a risk of an attack by a lone wolf terrorist, said Ms Valentina Soria, a counter-terrorism and security research analyst at London-based think tank - the Royal United Services Institute.
'Security around the Olympic stadium in London is really very, very good, so an attack there is highly unlikely,' she added.
A lone terrorist, however, is more likely to fall below the radar of the intelligence agencies. He may also pick targets such as shopping centres outside London because security there is less stringent.
'While it will not have the sort of impact as 7/7 or the 9/11 (terror attacks in New York City) , it will have a psychological impact because of the Olympics,' said Ms Soria.
The revelation that three of the four 7/7 bombers were British nationals, sparked a period of soul searching and cast a harsh spotlight on Britain's 1.6 million-strong Muslim community.
Questions of trust began to surface.
What spurred men born and raised in Britain to turn so horrifically against it? And more importantly, what could be done to stop future home-grown terrorists?
Initially, many in Britain's Muslim community began to feel demonised by the actions of a few, and were 'understandably defensive' when dealing with the authorities, said Ms Soria.
Since then, however, efforts by the State and police to make inroads with this often socially disadvantaged group have paid off.
Ms Soria added that Britain's South Asian community, where three of the 7/7 bombers came from, is better integrated with mainstream society than it was seven years ago.
'There are also more instances of people coming forward to co-operate with police to tackle the issue of extremism.'
At the same time, the security services have cracked down on terror cells in Britain. Notable successes include foiling a 2006 plot to blow up 10 transatlantic airliners departing from London.
But while the rhetoric in British mosques has become less angry and anti-western, new arenas for radicalisation have opened up, said Dr Usama Hasan, a senior researcher at Quilliam, a British Muslim think-tank that challenges extremism.
Threat from Somalia
The war on terror may have hobbled Al-Qaeda's terrorist training camps in Afghanistan and Pakistan, but the void is being filled by lawless parts of Africa.
Dr Hasan, 40, said eyewitness accounts suggest that 'hundreds' of British Somalis have been to terrorist training camps in Somalia run by Al-Shabaab, a terrorist organisation which merged with Al-Qaeda in February 2012.
Britain is thought to be home to at least 350,000 Somalis. Many fled their homeland in 1991 when president Said Barre was overthrown and the nation was gripped by violence as rival warlords vied for power. Clan divisions and lawlessness still blight the east-African country. Its transitional federal government has been locked in a battle with Al-Shabaab, which has controlled large parts of the country.
All this has made the nation a ready breeding ground of Islamist terrorists. Yemen, where Al-Qaeda in the Arabian peninsula is based, faces similar problems.
'Somalia and Yemen are what Pakistan and Afghanistan were in the 1990s and the noughties (after 2000),' said Dr Hasan. 'They are relatively easy to get to and there are many young people in Britain with connections there.'
Social exclusion is one of the factors which can fuel radicalisation, said Dr Hasan.
A 2009 government report found Somali-born migrants have the lowest employment rate of all migrants in Britain. Young British Somalis have said they find themselves at arm's length from traditional Somali culture while not fitting in with British society either.
Back from the brink
As part of his work at Quilliam, Dr Hasan is asked to mentor individuals identified by the police as being at risk of radicalisation. Despite coming from a well-off background in Britain, he once had extremist leanings himself.
At the age of 19, he left his home in London to spend two weeks in Afghanistan in 1990, training to fight communist forces. As a newcomer, he spent only a day on the front line, but the experience gave him a 'taste of Jihad'.
After returning to Britain, however, he began to study his religion more deeply and found that the Islamist doctrine – pitting Muslims against non-Muslims - was too 'narrow-minded'.
'I realised that there were also many Muslims who were being killed by other Muslims and that a lot of these conflicts were to do with complex local issues and being fought along sectarian, tribal and ethnic lines.'
The Cambridge University graduate, who is also a trained Imam, remains a devout Muslim and said the concept of mercy is central to the Quran. When he talks to men who are toying with violent extremist ideologies, the discussion often centres upon the interpretations of Islamic teaching, but Dr Hasan also shares his personal story with them.
'It gives credibility because they see that I am a person who knows where they are coming from,' he said. 'I tell them about my experiences. Sometimes they listen and it can help them to think differently.'