The sister of a British tourist whose body washed up off a Thai beach earlier this year told a BBC documentary she believes his death was not an accident and criticised the local investigation into the case.

Laurie Kelly, speaking to presenter Zara McDermott in the final episode of the three-part series Thailand: The Dark Side of Paradise, said her brother Regan, 28, had travelled to Phuket for a month-long holiday and attended a New Year’s Eve party in Patong. Regan, a building surveyor from Selsdon, south London, went missing in the early hours of Jan. 3 after CCTV footage showed him leaving a hostel following a confrontation, and a naked body was found face down in the water off Patong Beach the next morning at about 10 a.m. Police later confirmed the body was Regan’s.

Laurie told McDermott she believed the death "wasn't an accident" and cited what she described as a vital detail: when Regan's body was found he had a sock on his foot, which she said made it unlikely he had entered the water voluntarily to swim. Footage from the Mad Monkey Hostel, where Regan had been staying, showed him speaking for several hours with another man before the conversation became increasingly heated, the family has said. The pair were later seen on CCTV nearer the seafront, with Regan running and the other man following shortly afterwards; the video then cut off, according to the family, who said police supplied the footage to them.

Regan’s family reported him missing after their messages to him went undelivered and phone messages showed only a single tick, Laurie said. A watersports instructor told Laurie he had found the body, reached it on a jet ski and held Regan’s hand as he was brought ashore, she recounted in the programme.

Laurie said she travelled to Thailand with her father to try to learn what happened and described the family's frustration with Patong Provincial Police, who she said treated the case as an accident and discouraged the family’s own inquiries. She said they were allegedly told by tourist police to stop their private searching because "we are stirring up a hornet's nest." The tourist police have denied that account. The Patong Provincial Police did not return the production team's request for comment, the programme said.

McDermott's documentary, which examines the contrast between Thailand's tourist-facing attractions and a darker undercurrent of drugs, alcohol-fuelled violence and sex tourism, featured Regan's case as one of several in which British visitors died or were harmed while travelling in the country. In interviews Laurie described Regan as an experienced traveller who had shared photos and messages during the trip until he suddenly stopped responding.

The family said police had sent photographs to confirm Regan’s identity after the body was retrieved from the water. Laurie told McDermott she still has many unanswered questions about her brother's final hours and said the experience had changed her.

The documentary's airing has renewed public attention to the case and broader concerns about safety for young tourists in certain Thai resort areas. Officials in Thailand have in recent years faced scrutiny from foreign governments and families of victims over the handling of investigations involving visitors. The programme said requests for comment from the local police were not answered; the tourist police denied the family’s account that they discouraged private inquiries.

In the documentary Laurie concluded, "I believe my brother was murdered," and said she could not see how she would fully come to terms with his death. The family continues to seek answers about the circumstances surrounding Regan Kelly’s death.