The Independent Police Complaints Commission (IPCC), which is investigating the east London shooting, was set up to replace the Police Complaints Authority.
A new watchdog was demanded after Stephen Lawrence's death
That body had been criticised for allowing police to check allegations about one another. The integrity and accountability of such investigations was questioned.
The 1993 Macpherson Inquiry into the racist murder of Stephen Lawrence and Lord Scarman's inquiry into the Brixton riots both recommended the establishment of an independent body to investigate police complaints.
The IPCC, which was created under the Police Reform Act in 2002, has 100 investigators to look into claims of police misconduct and deaths in custody.
It looks at serious incidents involving death or serious injury, and allegations including those of serious or organised corruption, racism or perverting the course of justice.
It investigated the shooting of Jean Charles de Menezes by police at Stockwell Tube station, the day after the failed 21 July bombings.
While many of its first senior investigators did have a police background, others had experience in banking, insurance and social work.
According to its website, the IPCC's 17 regional commissioners "guarantee its independence and by law can never have served as police officers".
The IPCC had a budget of £23m in its first year (2004/5) and £29.1m its second (2005/6). It has begun work on 83 independent and 314 managed investigations into the most serious complaints.
In 2004/5 it upheld 239 public appeals - out of 768 considered to be valid - about the way complaints were dealt with by local police.
At the end of an investigation commissioners can decide whether it should be referred to the Crown Prosecution Service. The file will also go to the coroner's office.
IPCC chairman Nick Hardwick, former head of the Refugee Council, said: "We aim to increase confidence in the police complaints system by transforming the way forces handle complaints from the public."