By Gordon Corera
BBC Security Correspondent
Those held have been under surveillance for several months
The operation which led to the arrests across Birmingham was the product of six months of investigation by the West Midlands Police, supported by the security service MI5 and other agencies.
It was led by a new regional counter-terrorist unit which employed the full range of police and surveillance techniques.
Following the arrests, officials made clear that the group of men were not suspected of planning mass casualty attacks in the UK in the form of bombings similar to those witnessed on 7 July 2005.
Instead, sources suggested the men were suspected of planning to kidnap an individual and murder them, filming the act and posting it on the internet.
The individual is said to have been identified by the plotters and is thought to have been a Muslim member of the armed forces.
While the group was under intensive surveillance, developments led to the police deciding to intervene.
This may have been because they felt the group was about to act.
The challenge for police will be to see if they can now gather sufficient evidence to back up the intelligence suggesting a plot.
Intelligence can be fragmentary, partial and in some cases even wrong, as witnessed in the high profile raid on a house in Forest Gate, East London last year where no-one was charged.
That event had a significant impact on public attitudes towards anti-terrorism raids and on community relations.
As a result, police in the West Midlands have been keen to project confidence about the strength of the information they had, and also to emphasise the work they were doing to reassure the local Muslim community.
If the reports proved to be correct, it would mark the arrival in the UK of a tactic developed in Iraq and used to chilling effect with the murder of Ken Bigley and other hostages.
Around the time of Bigley's murder in 2004, officials were privately worried that the tactic of videotaped kidnappings could be transferred over to the UK.
If there really was an intention to carry out such an attack it will be another worrying development.