A Scotland Yard anti-corruption detective and three ex-officers arrested on suspicion of involvement in a bribery scam were released on police bail on Thursday.
The 45-year-old detective constable was questioned on suspicion of providing information about Nigerian fraudster James Ibori to private investigators in return for cash.
Two private investigators, aged 53 and 58, and another man were arrested in connection with providing the money after raids at the London offices of Risc Management Ltd.
Giving evidence to the home affairs select committee on Wednesday Mike Schwarz, a partner at Bindmans LLP, made reference to possible cash payments made by investigation firm Risc to sources of theirs who were "presumably police officers or those close to the investigation."
Records showed a number of payments amounting to about £20,000 over an eight or nine-month period, he said.
Risc was hired by a law firm representing Mr Ibori, a former governor in Nigeria who was jailed for 13 years last month after admitting fraud totalling nearly £50 million.
Mr Schwarz, whose firm represented one of Mr Ibori's co-accused, told MPs: "What I have seen in that case is serious illegality on behalf of private investigators Risc Management Ltd... which involves apparent corruption right at the heart of New Scotland Yard."
Committee chairman Keith Vaz welcomed the action, saying: "There must be zero tolerance to any kind of illegality in the police service, particularly the sale of information on criminal investigations."