An inquiry into claims of police corruption in the first investigation into the murder of Stephen Lawrence is expected to be complete by the summer.
Stephen Lawrence was attacked at a south-east London bus stop
The Independent Police Complaints Commission (IPCC) interviewed police officers about allegations made in a BBC documentary last year.
IPCC deputy chairman John Wadham said the inquiry had "made progress".
Mr Lawrence, 18, was stabbed to death in south-east London in 1993. No-one has been convicted of his murder.
Neil Acourt, his brother Jamie, David Norris, Gary Dobson and Luke Knight were arrested soon afterwards.
Three of the men were acquitted of murder after a private prosecution brought by the Lawrence family collapsed at the Old Bailey in 1996.
The BBC documentary, entitled The Boys Who Killed Stephen Lawrence, included claims by former detective Neil Putnam about ex-detective sergeant John Davidson.
Mr Putnam claimed Mr Davidson took bribes from Clifford Norris, the father of murder suspect David Norris.
Mr Davidson denied the "devastating and false" allegations and said he had not hampered the Lawrence investigation "in any way".
The IPCC launched the inquiry after the documentary was broadcast last July.
It has interviewed Mr Putnam but has not yet decided whether to interview Mr Davidson, who now lives in Spain.
Mr Wadham said: "We are doing everything within our power to find out the truth about these allegations.
"However, investigating corruption charges that are alleged to have occurred 13 years ago is extremely challenging."
As a result the inquiry has taken "longer than we had wanted", Mr Wadham said.
"Having said that, our inquiry has made progress, including examining related investigations conducted by other bodies and identifying relevant material."