Two former RUC detectives arrested yesterday in connection with an inquiry into the police investigation of the UVF murder of a young man in 1997 were released last night without charge.
They were detained by officers from the Police Ombudsman's office, assisted by the PSNI.
A third former detective, Johnston Brown, was questioned on his return from holiday last night in connection with the inquiry. Mr Brown was detained in conection with an alleged attempt to pervert the course of justice and misconduct in public office.
Nuala O'Loan's office has been examining the RUC investigation into the killing of Raymond McCord jnr in 1997 following claims that at least one of those involved was also an RUC Special Branch agent.
It emerged yesterday that the two men are also being questioned about the UVF murder in 1993 of Catholic woman Sharon McKenna, as well as allegations of attempting to pervert the course of justice.
The original investigation into the McCord case is widening into an examination of the former RUC's intelligence-gathering methods and collusion with loyalist paramilitaries.
Justin Felice, who heads Mrs O'Loan's team of investigators, said yesterday: "The arrests and the searches are part of a major investigation by this office into the complaint by Mr Raymond McCord (snr) who alleged that the police did not investigate the murder of his son correctly.
"This has proved to be a very complex investigation and has also involved us looking into the police investigation into the murder of Sharon McKenna.
"The two former officers are being held at separate police stations and are being questioned about a number of allegations including attempting to pervert the course of justice and misconduct in public office."
Johnston Brown said earlier yesterday he expected to be questioned. He claimed in The Irish Times last April that elements within RUC Special Branch protected a named UVF figure, linked with the McCord murder, to ensure he was not exposed as an informant, despite the fact that they knew he was associated with several killings of Protestants and Catholics.
He told the BBC yesterday he had already responded to requests for assistance from the Ombudsman's office in 2001 and subsequently.
"I co-operated fully with the Ombudsman," he said.
"There is nothing in this that I didn't make clear to the Ombudsman when I retired in 2001. When they asked to see me I went forward on a number of occasions . . . to help the Ombudsman get round Special Branch obstruction."
He said he did not fear arrest and would meet any allegations Mrs O'Loan wants him to face.
"I have nothing to fear," he added. The problem when you speak out in relation to these serious matters is, they can't get anybody else because nobody else is talking."