A former Scotland Yard detective was being questioned last night on suspicion of involvement in the murder of a man whose brutal killing he investigated 21 years ago.

Sid Fillery, once a Metropolitan Police detective sergeant, was one of five men arrested yesterday in connection with the murder of Daniel Morgan, a private detective, in March 1987.

Morgan, 37, was found dead in the car park of the Golden Lion pub in Sydenham, southeast London, with an axe embedded in his head.

The failure to catch Morgan’s killers has dogged the Metropolitan Police ever since. Five murder inquiries have been conducted, giving rise to allegations of corruption within police ranks and claims of a cover-up.

Mr Fillery, 61, was a close contact of the dead man’s business partner, Jonathon Rees, and one of a number of officers who moonlighted as security guards for Southern Investigations.

A year after the murder he was discharged from the Met on medical grounds and took Morgan’s position as co-director of Southern Investigations. The agency was wound up in 2003 and Mr Fillery now lives in Norfolk, where he is involved in the management of a pub.

Mr Rees, 53, was also arrested yesterday over the murder, along with three other men — James Cook, 53, Glenn Vian, 49, and Garry Vian, 47.

All five men attended a Central London police station where they were being interviewed. In a related development, a serving Metropolitan Police officer was arrested on suspicion of misconduct in a public office. Scotland Yard said that his arrest was not connected with the murder but with more recent events linked to the suspects.

Assistant Commissioner John Yates, who is in overall charge of the inquiry, said that the arrests were proof of “the Met’s continued determination to bring those responsible for this murder to justice”.

The drive to solve the case stems from the sense, at the highest levels in Scotland Yard, that the long failure to do so continues to taint the force.

The arrests come three years into the fifth police investigation into Morgan’s murder. The crucial breakthrough came last year when a “supergrass” approached police with evidence. The Times has learnt that the informant volunteered information about criminal activity, including a claim that he had been offered a substantial sum of money to kill Morgan.

It is understood that he has agreed to testify under the provisions of the Serious Organised Crime and Policing Act 2005, which formalised Queen’s Evidence arrangements in British law. It is the first time that Scotland Yard has used the new legislation. The Times has also learnt that in the past three months another witness has come forward.

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