A baffling litany of bad luck, tragic errors and panic in high places

The call that came to Scotland Yard's CO19 firearms unit at 5.05am on 22 July, 2005 was categorical. Armed officers were needed to support colleagues watching a flat in south London "as soon as practicable". Given that the apartment in question was the suspected address of two fugitive suicide bombers, it was expected that the team would move immediately.

The reality was tragically different. The members of Trojan 805 were not briefed about their role until four hours after that order. En route to their holding point, some of the officers had enough time to stop at a petrol station and fill their cars with fuel, unaware that, barely two miles away, events were unfolding that would shake Britain's largest police force to its foundations.

The delay in deploying a firearms unit to arrest anyone leaving 21 Scotia Road in Tulse Hill on the morning of 22 July was the first in a succession of errors, misunderstandings and confusions that resulted in two officers from the same team sprinting into a Tube train barely two hours later and firing seven bullets into the head of an innocent man.

The conviction of the Metropolitan Police for the "fundamental failures" that led to the death of Jean Charles de Menezes has revealed a detailed account of the fast-moving but irredeemably flawed operation.

At lunchtime on 21 July, a gang of four men tried to mount the second suicide attack on London's transport system in a fortnight. The failed attempt put enormous pressure on the police to catch the fugitives. No 21 Scotia Road, situated in a cul de sac, was placed on the front line of the investigation after one of the bombers' gym cards was discovered in a rucksack containing one of the failed bombs.

The address was that of Hussein Osman, who had tried to blow himself up at Shepherd's Bush station; it was also linked to Yassin Omar, another of the gang. At 4.55am on 22 July, Commander John McDowell, the officer in charge of the hunt for the bombers, laid out a plan to place the communal block under surveillance by Special Branch officers.

Crucially, Mr McDowell ordered that the surveillance team, which included a member of the Army's special forces on attachment, was to be supported by CO19 officers. Members of the armed team were to be concealed around the corner and stop everyone seen leaving the building, to confirm their identities.

If the plan had been correctly applied, Mr de Menezes would have been intercepted as he left for his work as an electrician in north London at 9.33am and, in all probability, his encounter with police that day would have resulted in nothing untoward.

In the event, officers from Trojan 805 arrived for work as usual at 7am in Aldgate, east London, and had selected their weapons and cars without being given details of their role. They arrived at Nightingale Lane police station near Clapham Common at 8.10am and were finally briefed between 8.45am and 9.15am.

As the Brazilian was leaving his home, the firearms unit was still in Nightingale Lane. "Alan", the Special Branch officer who made the call to CO19 at 5.05am to pass on Mr McDowell's orders, described the delayed deployment as "totally unacceptable".

Meanwhile, Room 1600 at Scotland Yard – the Met's state-of-the-art nerve centre designed to put senior officers in "real-time" contact with teams on the ground – was far from sedate. Prosecutors alleged that the operations room, where officers led by Deputy Assistant Commissioner Cressida Dick were directing the hunt for the four bombers, was "noisy and chaotic". People were shouting and at least one officer in charge of monitoring messages from Scotia Road was struggling to hear what was being said, it was alleged. In court, Ms Dick strongly denied the room was noisy, stating that she was told five times between 9.33am and 10.05am that the surveillance team believed Mr de Menezes was Osman and that his behaviour – appearing nervous, sending text messages, getting on and off buses – suggested he was a terror suspect. But, while Ms Dick may have been clear in her own mind what was happening, the level of clarity was not shared on the ground.

The Special Branch team began pursuing Mr de Menezes as he made his journey to Stockwell Tube station by bus. The trial heard that at no point in the 34 minutes between the 27-year-old leaving his home and the moments when he was shot did the surveillance officers believe they had formally identified him as Osman.

The question remains as to why an individual who was believed to be a suicide bomber was allowed to board two buses and a Tube train, despite being followed from his front door.

The absence of armed CO19 officers to back up the surveillance squad was becoming dramatically clear as orders were given that Mr de Menezes should be "stopped" or detained.

When an officer in Room 1600 was told the firearms unit could not carry out the arrest because it was still too far away, he replied: "Get yourself there."

Despite her denials, the chaos in the control room seems to have been epitomised by Ms Dick and the series of contradictory orders she issued in the moments before Mr de Menezes entered the Tube station– while CO19 officers were still on their way.

A log of the operation in Room 1600 shows how, in the space of four minutes, Ms Dick ordered the surveillance team to arrest the Brazilian, then directed that it should be done by the CO19 unit, then once more ordered the surveillance officers to intervene and, barely a second later, insisted that CO19 make the arrest after being told they were at the station.

The log then notes at 10.08am: "Subject has been shot. Ambulance ASAP."