Terrorists brought to justice at cost of 100m pounds

London: Abdullah Ahmad Ali, Tanvir Hussain and Assad Sarwar, the terrorists who plotted to cause death "on an unprecedented scale", were finally brought to justice after one of the most complex and costly criminal investigations since the Second World War.

The police and MI5 had carried out more than a year of surveillance work before the gang was rounded up, and it took a further three years, and two trials, to convict those who planned to blow up transatlantic aircraft using liquid-based bombs. In total, the investigation and trials are estimated to have cost £100 million (Dh600 million).

The first hint that a suicide bombing "spectacular" had been thwarted came on August 10, 2006, at the height of the holiday season, when airports were thrown into chaos as the authorities suddenly imposed a ban on liquids being taken on board aircraft. John Reid, the home secretary at the time, made an emergency statement to explain that the security services had arrested members of a suspected British-based Al Qaida cell that had aimed to smuggle bombs disguised as soft drinks on to aircraft.

It would later emerge that MI5 agents had been convinced that the plotters were in the final stages of planning a co-ordinated wave of attacks on aircraft that could have killed up to 5,000 passengers and as many again on the ground.

The origins of the plot went back to December 2004 when Abu Ubaida Al Masri, an Egyptian bomb-maker, was in the mountains of Pakistan holding classes in how to make explosives using hydrogen peroxide.

In Pakistan at the time was Abdullah Ahmad Ali. Ali, 28, was born in east London, into a religious family of eight brothers and sisters with close connections to Pakistan.

His only jobs were working part-time at Halfords and Wickes and most of his energy went into a charity called the Islamic Medical Association, based in Hackney, which he used as cover to travel to Afghanistan, where he drove an ambulance in refugee camps. He introduced several of his school friends to the charity, including Tanvir Hussain. Others he met through the charity included Assad Sarwar and Umar Islam, who also went on to become co-conspirators.

Investigators believe that during his time in Afghanistan, Ali was put in touch with bomb-maker Al Masri by Rashid Rauf, an Al Qaida fixer who had fled his native Birmingham after being sought in connection with the murder of his uncle.

In June 2006 Ali returned from Pakistan and MI5 began tapping his phone calls and emails, discovering he was in contact with someone in Pakistan and apparently talking in code.

On July 5, 2006, MI5 began the largest surveillance operation it had ever run, diverting all available resources to Operation Overt.

MI5's breakthrough came on July 26, when Ali was seen visiting an empty flat in Forest Road, Walthamstow. Agents broke in and planted listening devices and cameras.

Just over a week later, on August 3, Ali and his friend, Tanvir Hussain, were filmed drilling a hole in the bottom of a soft drinks bottle.

The jury at Woolwich Crown Court would later hear that this was to enable the gang to empty the bottle and replace its contents with liquid explosives without breaking the cap seal.

On August 6, the stakes were raised still higher as Ali was followed into an internet cafe in Walthamstow and seen looking up flight times. He copied details of seven flights that took off within two and a half hours of each other, meaning they would have all been in the air at the same time and the authorities would have been powerless to save any of them once the first had been blown up.

Throughout the investigation, the White House had been kept abreast of developments, and, after President George W Bush was informed of the suspects' latest moves, he was not prepared to wait any longer, and ordered the arrest of Rauf in Pakistan.

At Scotland Yard, where officers felt they still needed more evidence, the move caused panic because the suspects were in daily contact with Rauf. That evening Ali met Sarwar in a car park beside Waltham Forest Town Hall, where he handed over several suicide videos. The police immediately moved in to arrest them, and held a total of 24 suspects in a series of coordinated raids.

-The Telegraph Group Limited, London 2009