A report by an independent panel details several possible theories as to why the private investigator was killed.
One is that Mr Morgan was on the verge of revealing links between corrupt police officers and organised criminals.
In February 1987, he recovered a 4×4 vehicle from Malta that was connected to an ongoing fraud investigation in Yorkshire.
The report said: “Individuals linked with organised crime and allegedly to corrupt police officers were associated with a Range Rover recovered from Malta by Daniel Morgan in February 1987.
“West Yorkshire Police were investigating individuals suspected of committing major fraud and one of those individuals had removed the Range Rover which had been recovered by Daniel Morgan. That individual was on bail in London at that time.”
Mr Morgan had also mentioned to friends that he had information that he could sell to a Sunday paper for £250,000.
Business associate Bryan Madagan said that in May 1987, Mr Morgan had told him that he had been offered the money by a newspaper for an expose on how he got information.
Another theory was that the murder could have been linked to the death of police officer DC Alan Holmes, known as Taffy, who took his own life in July 1987.
It was suggested by Jonathan Rees and a former police officer called Derek Haslam that Mr Morgan and Mr Holmes wanted to sell information regarding corruption.
The panel found no evidence to back this theory.
Another possibility was the idea that police officers in south-east London were making money illegally selling information and moonlighting, for example as security guards, and felt that Mr Morgan threatened both the profits and their future careers.
The report said: “It is also possible that local officers involved in identified lucrative corrupt practices, such as selling confidential information, assisting criminals with inside police information and ‘moonlighting’, thought that their police careers and pensions were under threat, and that future, potentially lucrative options might be put at risk by Daniel Morgan’s alleged intention to reveal what he knew.
“The evidence supporting this theory as to why Daniel Morgan was murdered was never seriously investigated, despite the fact that in the years following Daniel Morgan’s murder, several of the police officers connected to Daniel Morgan’s circles and business were investigated for and convicted of serious crime.”
There was also an allegation that Mr Morgan’s business partner Jonathan Rees wanted to kill him.
Kevin Lennon, the bookkeeper for their firm Southern Investigations, claimed that Mr Rees had shouted at Mr Morgan, and spoken about getting police to stop him for drink-driving so that he would be forced to give up his half of the business, which had an outstanding tax liability of £23,400.
He said that Mr Rees had claimed that he would get his “mates at Catford nick” to help him, and that in 1986 he had told Mr Lennon that when Mr Morgan was dead, a police officer called Sid Fillery would take his place.
Other possible motives considered were Mr Morgan trying to identify a drug dealer who was selling illegal substances to a client’s daughter; a possible killing by the jealous husband of one of the women Mr Morgan allegedly had an affair with; and a dispute over the theft of a stereo from his car and threats made by the youths who had taken it.
Mr Morgan had also worked as a bailiff and repossessing cars, and the day before his murder had served a court summons on a man who had previous convictions for violent offences.