Officers in a police force that bungled the investigation into a suspected racist killing, allegedly went on holiday, dined and partied with suspects arrested over the attack, the Guardian has learned. The new claims come on the 25th anniversary of the death of Jay Abatan, 42, who was attacked outside a Brighton nightclub in January 1999. No one has been convicted for the fatal attack on Jay or the assault immediately afterwards on his brother, Michael. Both were set upon by a group of white men as they left the Ocean Rooms nightclub. Sussex police, which investigated the death of Abatan, who was mixed race, made a string of errors, revealed by hundreds of internal police documents seen by the Guardian. The documents show that witnesses and forensic opportunities missed, that police used racist terms such as “half-caste” and “coloured”, that officers failed to treat the death seriously enough, and that the investigation was under-resourced. An inquest ruled the incident an unlawful and unprovoked killing and said no convictions ensued despite suspects being identified quickly by Sussex officers committed to getting justice. Michael Abatan said a new force should take over the investigation and a public inquiry was needed: “I just want the truth and I am sick of being lied to and treated like a fool. I trusted the authorities to do their job and they let Jay down.” Jay, a tax accountant, was attacked and fractured his skull after falling and died five days later, the inquest found. Michael claims that before the group containing the attackers reached the club, they had been drinking with a police officer. Inside the Ocean Rooms, Michael said, there was tension after Jay danced with a white woman who was part of the group containing the attackers. The violence started when Jay’s group left the nightclub to get a taxi. A postmortem found that Jay suffered facial grazes, a fractured eye socket, another wound to his mouth, and that the back of his skull was fractured when he fell and his head hit on concrete. Michael said he saw his brother plummeting backwards and then at least two attackers turned on him, punching and kicking him to the ground for 15 seconds. He got up to find his brother on the floor. So rapid had the attack been that Jay’s left hand was still in his pocket and his eyes were open. Two men were charged with Jay’s manslaughter, but the case collapsed in June 1999 owing to a lack of evidence. Two men were acquitted the following year of the attack on Michael. Two other men were also arrested, but no charges were brought. All deny wrongdoing. The Guardian has heard allegations from a witness who was present that the suspects went on holiday together in August 1999 to a chateau in northern France. The witness said the attacks outside the club and the threat of prosecutions were discussed, and that the partner of a serving officer and his children were present at the chateau. Other sources said a serving Sussex officer joined the suspects later on the holiday, one of several alleged associations between police and the suspects that concern the Abatan family. The witness, who is fearful of reprisals, said: “They knew what they had done. They were coked up and pissed.” The witness said one suspect, Graham Curtis, agreed that he would take the blame to protect the others: “He said he would take the fall for the group.” Curtis killed himself in 2003. Another suspect within the hearing of the witness, allegedly called Jay, who was mixed race, a four-letter derogatory word for someone of Pakistani origin. The witness said they had been part of a social group with the suspects and, at a party at one of the suspect’s homes, there had been a glass bowl full of cocaine: “They used cocaine like others use icing sugar for baking.” In September 1999 at the Concorde nightclub in Brighton, the witness claims they saw a Sussex police officer with a suspect for the attacks. Further claims of connections between Sussex officers and the suspects come from Michael. In 2010 he said he saw an officer who had worked on his case having lunch in a cafe with one of the suspects on the Blatchington Road in Hove: “I was aghast. They were having lunch. There were plates on the table.” The Conservative MP Peter Bottomley, who has supported the Abatan family, said: “One or more people in the police have questions to answer about their links to the suspects, before, during and after the events outside the Ocean Rooms.” Sussex police have been told about all the alleged associations, Michael said. The Independent Office for Police Conduct said it had received no referrals from the Sussex force about allegations of unauthorised association between a serving officer and suspects for a serious criminal offence in the Abatan case. A source added that they would expect to. A senior policing source said claims about officers associating with suspects for serious offences should be a “red flag” and warrant further and urgent investigation as well as referral to the police watchdog. There were many potential witnesses in the area at the time of the attack, at about 3am on 24 January 1999. Among the errors identified by internal police reports were that Sussex police: reduced the number of officers on the investigation after Jay died; potentially missed “many” witnesses; wrongly said one witness saw nothing when in fact they did and had been so close that they had called an ambulance for Jay; failed to investigate the suspects and their potential links and history; failed to interview one witness for over a year. When the interview finally took place, the witness was able to name 11 other people present at the scene. Michael said: “They were very slow and reluctant to admit their mistakes and I felt pressured by Sussex police to stop questioning their failings.” Sussex police did not answer questions put to them about the alleged associations between some of their officers and the suspects. In a statement, they insisted they had changed for the better since 1999: “We had apologised publicly for the failings in 1999, but reinforce that current investigative practices are vastly different. We remain committed to investigating any significant new information that will assist in convicting those responsible for Jay’s death. “Senior detectives have met and corresponded with the Abatan family on a number of occasions over the years in order to respond to their concerns and to provide answers to any questions raised. “The force has completely updated and reorganised the way it investigates both major crime and critical incidents since 1999.” Bottomley believes the Abatan case has striking parallels to the Stephen Lawrence case in London. Jay died a month before the official report on the Lawrence scandal found police prejudice had helped to shield the killers. Michael said: “I believe the parallels with the Stephen Lawrence case are corruption … and institutional racism that meant a black man’s life was less important than preserving their old ways of doing things that protects criminals.” A vigil will be held outside parliament on Monday night to mark the 25th anniversary of Jay’s death. Police said anyone with information can contact them quoting Operation Dorchester, or Crimestoppers, which is independent of the police, on 0800 555 111.