Human rights groups are investigating a death at a Del Monte pineapple farm in Kenya after a man’s body was found in a dam there last month. The body of Peter Mutuku Mutisya, 25, was discovered floating in the dam on Del Monte’s plantation near Thika on 17 November, four days after friends said he had gone there to steal pineapples. The developments have been uncovered in a joint investigation by the Guardian and the Bureau of Investigative Journalism (TBIJ). They follow allegations published in June of brutal assaults and killings by security guards at the farm, which is the single largest exporter of Kenyan produce to the world. After the revelations in June, Del Monte committed “to constant improvements” to the way it operates “to adhere to the highest international human rights standards”. A postmortem commissioned by the police gave Mutisya’s cause of death as drowning and identified “no obvious external injury”. It was also attended by a doctor paid for by Del Monte who wrote his own report. The Guardian shared both reports and a photograph of the body with a leading British forensic pathologist. He said he believed the body had marks that could be signs of injury, including to the head and arm, and that missing details in the postmortem write-ups rang “lots of alarm bells”. A spokesperson for Del Monte, which is the world’s biggest pineapple supplier, said it had “fully cooperated with Kenyan authorities throughout its investigation last month” and offered its “heartfelt condolences to the family”. They added: “According to the postmortem report that was approved by the directorate of criminal investigations officer and four different doctors – all of whom were present during the postmortem – the individual died by drowning, and there were no indications of foul play.” The case is being investigated by the Kenya National Commission on Human Rights (KNCHR) and the non-governmental organisation the Kenya Human Rights Commission. Dr Bernard Mogesa, the chief executive of the KNCHR, said: “The case is still under active investigation by our complaints and investigation division and the Kenya National Commission on Human Rights continues to work diligently to uncover the truth, holding those responsible accountable for any unlawful actions.” The commission, which was established by an act of parliament in Kenya, announced a broader investigation into allegations of violence and killings at Del Monte in June after the publication of articles in the Guardian and TBIJ. It has yet to publish any findings. Brian Olang K’Olang, a programme officer investigating the case for the Kenya Human Rights Commission, viewed the body in the morgue and interviewed witnesses on the day he was found. He described seeing trauma to the back of Mutisya’s head and other injuries. In his view, they were obvious. “I believe there was foul play.” He fears the pathologist may have “deliberately ignored” the signs of injury. Martin Chege Mutuku, 27, claims to the Guardian that he and another friend went with Mutisya on 13 November to steal ripe pineapples. Mutuku said that they were leaving the pineapple field and Mutisya had gone on ahead when three guards appeared. He claimed he was caught, beaten and tied up on the ground. Court records show Mutuku was charged the next day with theft from the farm that night. A third man said he successfully ran away unharmed. Mutuku said that while he was on the ground, the guards heard Mutisya returning with sacks and two chased after him. “I heard a scream and then silence a few minutes after,” he claimed. Mutisya’s father, Samuel Katendie, saw his son’s body and said there were signs of bleeding from the back of the head and marks on the neck that looked like he had been strangled. Bodies can be damaged after death but all marks should be recorded in a postmortem. Mutisya’s father said his son knew how to swim well and that he could not understand how he would drown. “I lost hope for justice after the doctor said there was no injury,” he said of the postmortem conclusion on 22 November. A private consultant pathologist said he charged Del Monte to attend the postmortem for them. His report was emphatic saying: “No evidence of involvement of person (third party) into the death of this person since there were no injuries seen during postmortem examination.” A leading forensic pathologist working in Britain who did not want his name published said this was something no pathologist should say. He said omissions in both postmortem reports raised questions about whether they had been written to support the idea Mutisya had drowned. He and another British pathologist who analysed the reports noted that only the official postmortem report recorded petechial haemorrhages, pinprick bleeding around the eyes. This can be indicative of strangulation, though can have other causes, including drowning. They said that, at the least, this should have prompted a written note on an internal and external examination of the neck but this was not provided in either report. The Kenyan pathologist commissioned by Del Monte insisted he had drawn the right conclusions. “Yes, my report is very emphatic. This is the standard practice in forensic postmortem.” He questioned whether any pathologist could “make any meaningful comment” from a photograph of a decomposed body. A spokesperson for Del Monte said: “We believe in the Kenyan judicial system … Fresh Del Monte remains fully committed and supportive of the community in which it operates in Kenya, and we remain dedicated to our employees, our community, and our 135-year-old commitment to human rights. We have actively supported the community over the years and take our social responsibility very seriously there, as we do everywhere else where we operate.” The spokesperson said the Kenyan authorities were taking the lead on the investigation but nobody in the police responded to requests for comment.