At least two men who have connections to last week's London bombings are alive and still at large.
The first is a man, who was seen on surveillance tapes at Luton station, located outside of London, as he bid farewell to the four bombers the morning of the attacks. The other is Magdy El Nashar, an Egyptian chemist, who attended and received training at North Carolina State University.
British police think El Nashar may have helped the London group build their bombs before leaving England two weeks before the attacks. They have since issued a worldwide alert for him.
The picture shows Hasib Hussain, 18, at the Luton train station at 7:20 a.m., one week ago today. Two-and-a-half hours later, his backpack full of explosives was detonated. It killed him and 13 others on a crowded double-decker bus.
Now police are on the search for answers to how such a plot was carried out. "Who supported them? Who financed them?" asked Deputy Assistant Commissioner Peter Clarke, head of the Anti-Terrorist Branch of the Metropolitan Police. "Who trained them? Who encouraged them?"
Officials tell ABC News the London bombers have been connected to an al Qaeda plot planned two years ago in the Pakistani city of Lahore.
The laptop computer of Naeem Noor Khan, a captured al Qaeda leader, contained plans for a coordinated series of attacks on the London subway system, as well as on financial buildings in both New York and Washington.
"There's absolutely no doubt he was part of an al Qaeda operation aimed at not only the United States but Great Britain," explained Alexis Debat, a former official in the French Defense Ministry who is now a senior terrorism consultant for ABC News.
At the time, authorities thought they had foiled the London subway plot by arresting more than a dozen young Britons of Pakistani descent last August in Luton, a city known for its ties to terrorism.
"For some time, the locus of terrorism in Britain has been around the Luton area and in some of the northern cities," said Michael Clark, professor of defense at King's College in London.
Security officials tell ABC News they have discovered links between the eldest of the London bombers, Mohammed Sadique Khan, 30, and the original group in Luton. Officials also believe it was not a coincidence the subway bombers all met at the Luton train station last week.
"It is very likely this group was activated last year after the other group was arrested," Debat said.
One of Khan's friends informed the BBC today that Khan had undergone training for explosives at terror camps in both Pakistan and Afghanistan. This piece of information only strengthened the London-Pakistani connection.
ABC News' Rhonda Schwartz, David Scott, Jill Rackmill, Madeleine Sauer and Simon Surowicz contributed to this report.