Bomb plot: What we know... and what we don't

What we know

1 Twenty-three young Muslims are being questioned about an alleged plot to blow up passenger airliners flying between Britain and the United States. They are aged between 17 and 35 and come from east London, Birmingham and High Wycombe. Most were named by the Treasury when it froze their assets. None has been charged.

2 Pakistani police have arrested several men accused of links to the British plot. They include Rashid Rauf, who is related to one of the men detained in Britain and is said to be a key suspect.

3 Britain's level of threat was raised for the first time to "critical", meaning an attack is considered imminent, early on Thursday morning. Ministers say it will remain at the level for the time being, to "err on the side of caution".

4 Several meetings of Cobra, the Whitehall civil contingencies committee, have been held with John Reid in the chair. Tony Blair is said to be keeping in regular contact from his holiday retreat in Barbados, but he has decided not to return to the UK for the time being.

5 Stringent new security checks have been introduced in British airports, with travellers only allowed to carry a bare minimum of hand luggage in plastic bags and facing body searches. There has been chaos at many UK airports.

and what we still don't know

1 The exact nature of the alleged plot, including which chemicals were being used and how they would be detonated. Security sources believe nine or 10 planes would have been blown up in three phases, each by a suicide bomber on board. Other reports place the number of targets at between five and 12. There has been speculation the planes could have been blown up above the UK or the US to cause extra carnage on the ground; alternatively, they could have been destroyed above the Atlantic to make it more difficult for investigators to establish what happened. The most likely scenario, however, is that the bombers would have detonated their devices at roughly the same time regardless of their location. Police have said "items of interest" have been discovered in, or near, the suspects' homes. They have variously been reported to be martyrdom videos, a trigger device and bottles.

2 How the alleged conspirators were put in contact with each other. Were they radicalised in Britain? Or did some embrace extremism after travelling to Pakistan, where five of them are said to have learnt bomb-making skills at al-Qa'ida training camps. Both Rashid Rauf and Matiur Rehman, a senior al-Qa'ida operative in Pakistan, are reported to have been involved. However, security sources are convinced that this is largely a British-based plot controlled by UK citizens alleged to have close al-Qa'ida links.

Is there a link with the July 7 attacks? So far none has been established, although some of the suspects were said to have been in Pakistan at the same time as the suicide bombers Mohammad Sidique Khan and Shazad Tanweer. Who masterminded the operation? According to one report, one of those in custody is the British leader of al-Qa'ida, although the disparate nature of the terrorist networks means that there is no one "Mr Big" in this country.

3 How many co-conspirators are still in circulation? John Reid has said he believes all the main suspects are in custody, but there have been reports that at least two suspects escaped from last week's raids. There are fears that the foiled alleged plot could prompt sympathisers into dramatic action.

4 What happened on Wednesday to accelerate the raids? Initially it appeared the alleged co-conspirators were believed to be about to board their target planes. But we now know that the arrest of Mr Rauf in Pakistan led the police to carry out the dawn arrests hours later for fear his detention could force the cell to take premature action. What is not clear is why he was arrested; it may have been a mistake by the Pakistani security services or he could have been picked up for a criminal arrest. Either way his arrest caused a crisis for the British police.

5 The extent of Tony Blair's knowledge. Downing Street had said Mr Blair did not know the raids would be launched on Wednesday night, so felt safe to fly to Barbados the day before. Why then did he believe the surveillance operation was sufficiently significant to brief George Bush about it the previous Sunday? And why have we barely seen John Prescott, who is meant to be charge of the Government in Mr Blair's absence?