A former Metropolitan police detective has been arrested over the murder of a man whose death he himself investigated.

The arrest was one of six made today in connection with one of Britain's most controversial and high-profile unsolved killings.

Five men were arrested on suspicion of the murder of Daniel Morgan, a 37-year-old private eye who was found dead with an axe in his head in a south London pub car park in 1987.

A sixth man, a serving Met police officer, was arrested on suspicion of misconduct in a public office.

The latest arrests, which result from a fifth investigation into the killing, are partly the result of witnesses - some from the criminal underworld - coming forward with statements after many years.

It is understood recent advances in DNA and changes in the law regarding the giving of evidence against fellow criminals have played a part in the inquiry.

Sid Fillery, 61, a former detective sergeant attached to Catford police station, in south London, who is now believed to be involved in running a pub on the Norfolk Broads, was arrested today in connection with the murder.

Jonathan Rees, 53, a freelance investigator and Morgan's former partner, was also believed to be held.

The others arrested were understood to be Jimmy Cook, 53, Glenn Vian, 49, and Garry Vian, 47, all of south London. A 24-year-old serving Metropolitan police constable was arrested over allegations of misconduct in a public office.

The men were arrested when they answered bail at Charing Cross police station or attended by appointment. The police constable, who was not named, was arrested at his place of work. He has been suspended pending the investigation.

The murdered man's brother, Alastair Morgan, who has fought tirelessly for more than two decades to have the case brought to justice, welcomed the news.

"This has been an extremely long and arduous battle for my family but we hope that this will perhaps be the beginning of the end," he said.

The latest investigation, headed by Assistant Commissioner John Yates, had a team of 36 working re-examining tens of thousands of pages of statements from the original inquiries.

It was relaunched three years ago and located in a different building from Scotland Yard to avoid any suggestion of impropriety because of allegations of police corruption from the start of the case. Police are still seeking other witnesses and a £50,000 reward remains on offer.

"It has been over 21 years since Daniel's murder," Yates said today. "Since that tragic event there have been numerous investigations into his death none of which have been able to establish who was responsible.

"The one consistent theme throughout has been the astonishing determination of the Morgan family to ensure that those responsible are brought to justice. The current investigation has shown that there are still people out there who have highly pertinent evidence about this case. The arrests today demonstrate the Met's continued determination to bring those responsible to justice."

Yates added: "I also hope that these arrests are a reassurance to those in the community who also have information or evidence about what happened to Daniel. We have long said that within south London the identity of those responsible for the murder is one of the worst kept local secrets. I urge anyone with any information, however small, to get in touch with us."

Daniel Morgan, the son of an army officer, studied agriculture and then worked as a salesman in Denmark and as a travel courier before joining a detective agency.

He set up his own company, Southern Investigations, in 1984 with Rees. He had a number of successes in the field, tracking down an international fraudster for the Bank of Australia, for instance, and his company was financially successful: he owned a BMW, an Austin Healey 3000 – which two decades later was to play a part in the re-investigation – and a Jaguar. He was married with two children, now both grown up.

Rees had close contacts with the local police and his colleagues believed he wanted to be a policeman himself. Morgan was unhappy about the closeness of some of those relations and at the direction the work was taking. At the time of his death, it is believed Morgan was about to expose a south London drugs network, possibly involving corrupt police officers.

Tension between Morgan and Rees heightened over a Southern Investigations contract with the local Belmont car auction in Charlton, whose takings the firm had responsibility for depositing. In early 1987, Rees claimed to have been robbed of £18,000 of the takings, but Morgan had doubts about whether such a robbery had taken place. There were also said to be tensions between the two men over a woman they both knew.

On March 10, Morgan agreed to meet Rees at the Golden Lion in Sydenham, not their regular local but one that happened to be inside the Catford police area where friends of Rees worked. After the meeting, Morgan went outside to the pub's car park where he was struck on the back of the head with an axe and killed.

The axe, which had been bound with sticking plaster to avoid its bearer leaving fingerprints, was left embedded in his head. His £800 Rolex watch was missing but £1,100 was in his pocket, from which DNA traces have been found.

At the inquest, Rees suggested Morgan had made passes at the wives of clients and that there might be a motive for murder there. There were also tales that he had been watching Colombian cocaine dealers on behalf of a public figure whose daughter had become addicted. Detectives now believe that the real motive for the murder was a culmination of a number of factors.

On the original investigating team was Detective Sergeant Sid Fillery, a close friend of Rees who moonlighted for Southern Investigations and, after leaving the police, joined the firm. He did not initially declare his relationship with Rees.

The first investigation petered out and by the time Morgan's body was finally released for burial in August 1988, there had been no arrests.

Had it not been for Morgan's brother and mother, the case might have been logged as one of Scotland Yard's unsolved murders. But Alastair Morgan has been dogged in his pursuit of the case, lobbying politicians and police officers even when it seemed the case was closed.

After his first intervention, the Police Complaints Authority - now the IPCC - asked Hampshire police to reinvestigate. Three people were arrested in 1989, but charges were dropped. Two police officers who were arrested in connection with the inquiry have since been completely vindicated and there is no suggestion they were involved in any way.

The investigation is the first in which the Met police used the Serious and Organised Crime and Police Act (Socpa) of 2005 which provides a framework through which people can turn Queen's evidence against co-conspirators and other criminals.