Trail of suspicion that encircled the world

Who has been arrested during the recent anti-terrorist operations?

Thirteen men of Asian origin were arrested on Tuesday during a series of raids in London, Blackburn, Luton, and Bushey in Hertfordshire. One man was released hours later without charge. Among the remaining 12 is a man believed to be a high-ranking al-Qa'ida member known as Abu Eisa al-Hindi, codenamed Bilal.

Earlier, on 13 July, an alleged al-Qa'ida operative and computer expert called Mohammad Naeem Noor Khan was arrested in Pakistan. This led to the capture in Pakistan on 25 July of Ahmed Khalfan Ghailani, wanted for his alleged role in the 1998 bombing of two American embassies in Africa.

On Thursday night, a British man, Babar Ahmad, 30, was arrested in London following a request from the US for his extradition to face terrorism charges.

Were the Pakistan arrests linked to the London raids?

Yes. MI5 had been investigating Mr Khan for some months and had several of his contacts in Britain under surveillance, including Bilal. When details of Mr Khan's arrest was made public, the police were forced to arrest the suspected cell members in the UK.

So was there a plan for an imminent attack on Heathrow airport?

Authoritative British counter-terrorism sources have denied this to The Independent. They say photographs and details of Heathrow were found on a computer belonging to Mr Khan, but they were old and there was no specific plot.

Pakistani and US security sources have been quoted by other newspapers saying Mr Khan had updated detailed plans to use a lorry or car bomb at Heathrow. One set of sources must be wrong or the foreign intelligence has yet to pass on the information to the British authorities.

What is the link with the terror alert in the United States at the beginning of the week?

Officials initially said the two men arrested in Pakistan had revealed that operatives in the US had been carrying out detailed surveillance on five financial institutions. The Bush administration was accused of electioneering when it later emerged that the surveillance information was up to four years old.

Officials now insist there is new intelligence - some of it based on information provided by an al-Qa'ida suspect in British custody. There are also reports that Bilal is suspected of having written the surveillance reports on the five buildings.

Who is Bilal?

He is described by security sources as a major player. He has been living in London as head of a suspected terrorist cell and is thought to be a British citizen in his late 20s who regularly travels to Pakistan. Accused of drawing up details of possible targets.

Why are there so many contradictory reports?

Al-Qa'ida is a loose-knit organisation that is hard for Western agencies to infiltrate. Much of the information on the current operation comes from Pakistani intelligence sources who are considered very unreliable. Politicians and the police are reluctant to discuss security matters, which allows the media to interpret any information unchallenged.