Winning control of the inquiry into the death of Jean Charles de Menezes was an "important victory" for the Independent Police Complaints Commission, it said.
Jean Charles de Menezes was shot at Stockwell Tube
The most high profile case since it began work in April 2004, the IPCC wanted to underline its independence.
It was set up to replace the Police Complaints Authority, which 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 84 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.
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 and has begun work on 57 independent and 202 managed investigations into the most serious complaints.
It has upheld 277 public appeals - out of 833 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."
The IPCC promises to use its "authority and powers unflinchingly to help those who suffer injustice because of an abuse of police powers".
Although it has already dealt with many complex cases, the death of Mr Menezes has increased attention on the IPCC.
In addition to saying police "initially resisted" the investigation into the shooting of Mr Menezes in Stockwell, it was critical of the Home Office for announcing that his visa had expired two years ago.
Mr Hardwick said: "It's entirely irrelevant information. I'm rather surprised the Home Office should issue it."
But the workings of IPCC have also come under scrutiny, not least following the leak to ITV News of papers relating to the investigation into Mr Menezes' death.
Sources said on Thursday night that a member of the IPCC secretarial staff had been suspended during an inquiry into the source of the leaked papers.
The IPCC refused to comment, but said: "We will not be distracted from our thorough, professional and impartial investigation into the death of Mr de Menezes."