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.