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British special forces soldiers took part in the operation that led to the shoot-to-kill death of an innocent Brazilian electrician with no connection to the London bombings, defence sources said last week.

Jean Charles de Menezes was tailed by a surveillance team on July 22 as he caught a bus to Stockwell Underground station in south London. He was shot eight times when he fled from his pursuers at the Tube station.

The Ministry of Defence admitted last week that the army provided "technical assistance" to the surveillance operation but insisted the soldiers concerned were "not directly involved" in the shooting.

Press photographs of members of the armed response team taken in the immediate aftermath of the killing show at least one man carrying a special forces weapon that is not issued to SO19, the Metropolitan police firearms unit.

The man, wearing civilian clothes with a blue cap marked "Police", was carrying a specially modified Heckler & Koch G3K rifle with a shortened barrel and a butt from a PSG-1 sniper rifle fitted to it - a combination used by the SAS.

Another man, dressed in a T-shirt, jeans and trainers, was carrying a Heckler & Koch G36C. Although this weapon is used on occasion by SO19 it appears to be fitted with a target illuminator purchased as an "urgent operational requirement" for UK special forces involved in the war on terror.

The soldiers who took part in the surveillance operation that led to de Menezes's death included men from a secret undercover unit formed for operations in Northern Ireland, defence sources said.

Known then as 14 Int or the Det, it is reported to have formed the basis of the Special Reconnaissance Regiment, the newly created special forces unit stationed alongside the SAS at Hereford. The men include SAS soldiers serving on attachment and are part of a team of around 50 UK special forces that has operated in London since the July 7 bombings in which 56 people died.

Special forces counterterrorist experts have been regularly used to support police at Heathrow since the September 11 attacks. They moved into London a day after the July 7 bombings and have been supporting the police and gathering intelligence to help snare the suspects.

Members of SO19 (technically known as CO19) are trained by SAS and SBS instructors. One key tenet of that training is to ensure that a suicide bomber is killed rather than wounded, which would allow them to trigger a bomb.

The use of multiple shots to the head is the modus operandi of the special forces, whether from the SAS, the SBS or the undercover intelligence operators used in the Stockwell operation. Over the past 30 years the SAS has developed a reputation for never allowing gunmen to remain alive, an attitude shown most graphically during the 1980 Iranian hostages siege and the Gibraltar IRA killings eight years later.

"It is vital to strike fear into the minds of the terrorists," one former SAS officer said. "In an ongoing situation such as we have now the fear must be directed to the fact that we are watching them and will eventually (get) them. They need to know that they cannot escape.

"We know they are happy to kill themselves but that doesn't mean they are happy to be killed by others. As long as they evade the police they will think they are in control but the minute they are intercepted they lose control."

The Ministry of Defence insisted last week that the military involvement was limited in the operation that led to de Menezes's death. "We would describe it as technical assistance as part of a police-led operation under police control," a spokeswoman said. "It is a particular military capability that the police can draw on if needed. It was a low-level involvement in support of a police-controlled operation."

The Det is made up of the army's best urban surveillance operators using skills honed in Belfast against republican and loyalist terrorists. Its speciality has always been close target reconnaissance: undercover work among civilians, observing terrorists at close quarters, and carrying out covert searches of offices and houses for information and weapons.

The unit was very egalitarian when it operated in Northern Ireland. An operator's rank was always regarded as less important than his or her capabilities; it was also the only UK special forces unit to use women.

The Det broke into homes to gather intelligence and plant listening devices or hidden cameras. Weapons were left where they were found but "jarked" with tiny transmitters placed inside them that would provide warning should they be moved.