How an apparently straightforward arrest became a murder inquiry linked to al-Qa'ida

On Tuesday, at his office in Queen Anne's Gate in London, David Blunkett authorised a warrant for the arrest of a 23-year-old Algerian man under the Terrorism Act, which permits the detention pending deportation of any foreign national suspected of terrorism.

Unbeknown to the Home Secretary, his signature would set in train a series of events that would culminate in the fatal stabbing of a police officer and the arrest of a man alleged to be a key al-Qa'ida suspect responsible for his death.

That afternoon, DC Stephen Oake was among a group of officers in Manchester preparing to follow through Mr Blunkett's instructions. At 4.22, a team of tactical aid group (TAG) officers, wearing bullet-proof vests and probably carrying batons, burst into the first-floor flat at 4 Crumpsall Lane to make it safe for Special Branch officers to follow them.

Although they were surprised to discover not one but three North Africans in the flat, this appeared to create no immediate problems. Within 10 minutes, all three had been arrested and contained in one room of the flat, allowing DC Oake, two Special Branch colleagues and a member of the Metropolitan Police's SO13 anti-terrorist branch to enter with an immigration official.

During the ensuing interrogation, what became clear was that the unexpected North Africans ­ aged 27 and 29 ­ were possibly more important suspects than the man the officers had come to detain.

According to Greater Manchester Police's new Chief Constable, Michael Todd, the interrogation raised issues that made a "change of tack" highly necessary. When the Met's anti-terrorist officer checked the 27-year-old's details with London, the man was said to be a key al-Qa'ida suspect wanted by MI5 and Scotland Yard. The 29-year-old was identified as being wanted in connection with a suspected chemical plot.

"It was decided that we were going to conduct a forensic examination and preserve the evidence on the suspects themselves," said Mr Todd. This meant putting the three in white forensic suits and "bagging" their hands with plastic.

In the course of this procedure ­ at 5.45pm, nearly an hour and 20 minutes after the officers entered the flat ­ the 27-year-old is alleged to have become violent. With his colleagues looking on, the man is said to have broken free from the uniformed TAG officer who was guarding him, prompting a scuffle during which they ended up in the kitchen ­ within reach of a large kitchen knife. The North African then allegedly seized it and went berserk.

Instinctively, the Special Branch officers came to the uniformed officer's help. DC Oake's superior, a detective inspector, was stabbed in the chest, though the wound did not penetrate deeply; a 41-year-old detective sergeant was cut badly on the arm, the knife severing a main vein, and two TAG officers suffered leg injuries, one breaking his ankle.

DC Oake suffered a deep wound to the chest. Despite urgent medical help outside the flat, he was pronounced dead on arrival at North Manchester General Hospital, a mile away.

Attention switched to the chance that the flat ­ and therefore anybody who had been inside it ­ might be contaminated with a chemical agent. Ricin was an obvious possibility, because of last week's discovery of a plot to manufacture the substance in north London, but none was found. As a precautionary measure, the entire street was sealed off as police scoured the flat for evidence. North Manchester General Hospital also had to be closed to routine or emergency admissions until 2am.

Yesterday, the 29-year-old North African was taken to London for questioning by the Metropolitan Police. The 23-year-old was being questioned by immigration officials, while the 27-year-old was under investigation by a murder inquiry team, headed by a Greater Manchester detective chief superintendent.

Amid his grief yesterday, Stephen Oake's father, Robin ­ himself a former Greater Manchester assistant chief constable ­ raised some searching questions about the planning for the anti-terrorist operation. Crucially, he was puzzled about why the Special Branch officers had not worn body armour for a raid on the home of a suspected terrorist. "It is issued and you need to wear it," he said.

The answer may lie as much in the Special Branch culture as in the tactical planning for the raid. Though more uniformed officers than before now wear body armour on routine patrol, plain-clothes Special Branch officers still consider themselves detectives apart. Generally, they are still disinclined to encumber themselves with stab-proof body armour for discreet intelligence work.

In the aftermath of the tragedy, Jan Berry, the Police Federation chairwoman, indicated the culture may have to change and that body armour was imperative for all officers. "With new risks come new protections," she said.

GMP Assistant Chief Constable Alan Green admitted that his force's internal review needed to find "some answers". But he defended the decision not to handcuff the men. "There was a need [to] maximise the forensic operation. That is one of the reasons why they were not handcuffed. It's not basic procedure to handcuff," he said.

Meanwhile, the inhabitants of the multicultural Crumpsall district awoke yesterday to the astonishing fact that the ricin terror that they considered confined to London had materialised in their midst.

"We have had a lot of immigrants move into this street, but you can never imagine this. It's a harmonious place," said Abdul Khan, 74. By mid-morning there was already evidence that the raid had not helped racial harmony. At least one public dispute between a Jew and an Arab ensued. It centred on how a group of Islamic fundamentalists, including a local member of the hard-line al-Muhajiroun, had appeared at the scene on Tuesday night,apparently intent on makingpolitical capital from it.