Metropolitan Police officer Dave Cook was arrested on suspicion of misconduct in a public office last week.
He was arrested after Met detectives working on Operation Elveden passed information to the Independent Police Complaints Commission.
The operation is investigating alleged payments to police officers by newspapers.
It is not clear why the Met did not carry out the arrest and instead passed the inquiry on to the police watchdog.
Cook was on the police team investigating the 1987 murder of Daniel Morgan. In June 2002, Cook appealed on TV for fresh information on the murder.
Alex Marunchak from the News of the World (NOTW) had run a “surveillance” operation on behalf of private investigator Jonathan Rees and police detective Sid Fillery.
The two were suspected, and later cleared, of murdering Morgan.
Marunchak paid £150,000 of News Corp’s money to Rees for information.
When the Yard asked NOTW editor Rebekah Brooks why the paper was harassing Cook, she said they were investigating allegations that he was having an affair. The woman in question was his wife.
A raft of police officers took money to get evidence for the tabloids.
The scale of systematic use of bribery by the cops is yet to be fully revealed.