Private investigator Daniel Morgan had spent a spring evening drinking with his business partner in the Golden Lion pub in south London.

Less than an hour later, the 37-year-old was found dead in the car park with an axe embedded in his head.

That was in 1987. Now, 21 years on, no-one has been convicted of his murder despite the police suggesting the identity of those responsible is one of south London’s “worst-kept secrets”.

So did the crime truly stump Scotland Yard’s finest or was there something more sinister afoot?

Officers arrested

The police’s failure to solve the case has led to allegations of corruption and claims of a cover-up.

Mr Morgan’s links with the Metropolitan Police certainly ran deep. As co-owner of a firm called Southern Investigations, he employed off-duty police officers.

And in a statement, the Metropolitan Police say they believe Mr Morgan had been about to expose a south London drugs network, possibly involving police officers.

But to date, there has been no evidence to prove any cover-up or corruption.

There has also been no shortage of investigations into Mr Morgan’s murder - five in total.

The first two in the late 1980s proved inconclusive and a third was apparently carried out without Mr Morgan’s family being informed, despite their persistent lobbying.

In 2002, the Met Police offered a £50,000 reward to catch the killer and appeals were made on television.

Despite all such efforts, arrests were made, some people were charged but no-one was ever convicted.

Among those charged was Mr Morgan’s business partner but the charges were soon dropped.

Several serving police officers were also arrested but were exonerated. Two were later paid damages for false imprisonment.

In late 2002, there were further arrests in London and Croydon, followed by the arrest of a former police officer on suspicion of “misconduct in a public office” relating to the initial murder inquiry.

But the Crown Prosecution Service concluded there was insufficient evidence to mount a prosecution.

Now the police say two new suspects have been arrested and three others are being re-interviewed.

‘Professional hitman’

The news has been given a cautious welcome by Mr Morgan’s family who are hoping a breakthrough could mean an end to their “long and arduous ordeal”.

They believe a professional hitman was behind Mr Morgan’s death, as his £900 Rolex watch was missing but £1,100 in cash was left in his pocket.

The murder weapon was also found wrapped in two strips of plaster tape - apparently to avoid leaving fingerprints.

Since Mr Morgan’s death his brother, Alistair, has been doggedly pursuing justice.

“The circumstances surrounding Daniel’s death were so appalling you felt that there was such a cloud over the investigation and cloud over the Metropolitan Police, I felt I could not move on. I had to do something,” he said.

He described how the police’s attitude had gone from paranoid secrecy surrounding the early investigations to openness and trust in recent years.

Even after the fourth investigation, he did not give up and contacted the Metropolitan Police Authority - a move he says was partly responsible for the fifth investigation being launched in 2006 by Scotland Yard.

He says he now hopes new witnesses who have come forward will prove crucial.

Assistant Commissioner of the Met John Yates, who is in overall charge of the re-investigation, said the latest arrests demonstrated the Met’s continued determination to bring those responsible to justice.

He called for anyone with information to come forward, saying there were still people out there who have “highly pertinent evidence about this case”.

“We have long said that within south London, the identity of those responsible for the murder is one of the worst-kept local secrets,” Mr Yates said.

Police will be hoping that secret will be spilled before investigation number six.