The Metropolitan Police chief has tried to "reassure Londoners" over claims police tried to smear the family of Stephen Lawrence.
Undercover officer Peter Francis said he posed as an anti-racism campaigner to hunt for "disinformation".
Met chief Sir Bernard Hogan-Howe responded: "If things weren't being done properly, people have to be held to account.
"You can be assured we are going to pursue it rigorously."
Black teenager Stephen, 18, was stabbed to death in an unprovoked attack by a gang of white youths as he waited at a bus stop in south-east London in April 1993.
A number of suspects were identified soon after the attack but it took more than 18 years to bring his killers to justice. An inquiry accused the police of institutional racism and found failings in how they had investigated the murder.
The lawyer who represented the Lawrence family, Michael Mansfield QC, has called for a Leveson-style inquiry to investigate the ethics of the police.
Home Secretary Theresa May has said the claims will be examined by two existing inquiries.
When asked by BBC News how he can reassure Londoners the force has changed in the 20 years since Stephen Lawrence's murder, Mr Hogan-Howe said: "I took over the Met about two years ago and we looked immediately at the types of undercover operations we were running.
"We got an independent force to look at whether we were running according to the law, according to good faith and that we were doing things properly."
"I was reassured at the time that was what we were doing and we've kept reviewing that," the Met Police Commissioner added.
"I'm confident that what we're doing now is entirely according to the law and it is targeting serious and organised crime; it is targeting terrorists."