British investigators are trying to find out whether last week's failed bombings in London and Glasgow, Scotland, were hatched by a small group of medical professionals in the United Kingdom, or directed by an outside group, such as al-Qaida.
On Wednesday, when Britain reduced its terror threat level from "critical" to "severe," it sent a signal that the authorities were confident they had arrested the key suspects in the bomb plots.
With eight people in custody, British intelligence officials are now focused on some of the most fundamental facts of the case — who the suspects are and what allegedly drove them to plan the terror attacks.
Unlike other recent terror plots in Britain, the people involved in last week's attempted bombings were foreign and well educated, skilled medical professionals.
Sir Ian Blair, the head of London's Metropolitan police, said the group did not fit the mold.
"One of the things we've found with the al-Qaida-affiliated terrorism is that the tactics change all the time. But this is a new in the sense of a different group of people than we've had before," Blair said.
The suspects come from several countries in the Middle East — Iraq, Jordan and Saudi Arabia. Two of them come from India. They came to work in the United Kingdom at different times over the past few years. British security forces are trying to build a detailed picture of how their lives interconnected. Did they go to school together? Did they have some kind of family tie?
"Were these people coming in from the outside to the United Kingdom in order to commit terrorist acts? Or, were they perfectly ordinary people coming to the United Kingdom to undertake medical practice and studies as doctors, who were radicalized here?" asked Anthony Glees, with the Brunel Centre for Intelligence and Security Studies.
The question is, if they were recruited, who recruited them? Police are said to be particularly interested in one of the suspects, Bilal Abdullah. He was born in Britain, but he grew up in Iraq, where he received his medical degree. Abdullah worked at a hospital near Glasgow for less than a year, and his family and colleagues said he is a fervent Muslim.
One friend, Shiraz Maher, told the BBC about an incident when another Muslim man did not pray enough.
"He put this DVD on of Abu Musab al-Zarqawi beheading one of the hostages, and he said, 'Look, if you don't, this is what we do. We slaughter,'" Maher recalled.
British security officials say that Abdullah's alleged contact with radicals in Baghdad raises the possibility that al-Qaida in Iraq may have been behind the plot.
But analysts said it is possible some, or all, of the suspects drew inspiration and direction from other groups.
M.J. Gohel, an analyst with the security think-tank the Asia-Pacific Foundation, said experience with previous terror investigations showed how difficult it was to penetrate these groups, particularly those based in Afghanistan or Pakistan.
"We've never been able to identify or locate these mystery men, these people who have been the guiding hand who have provided the inspiration, maybe the finance, the training, etc.," Gohel said. "Those people are still out there, and they can, of course, recruit others."
The names of several of the suspects were on a British intelligence database, not as known jihadists, but because they had communicated with some extremists.
Garry Hindle, with the Royal United Services Institute, said he doubts the suspects were part of a broader, sophisticated network.
"If they are a cell that have come to the U.K. with a purpose of carrying out this attack, why were they not more proficient? Why did they fail?" Hindle questioned.
So far, none of the suspects has been charged. They can be held for up to one month while police carry on with their investigation.