Hesbon Amadade, the man accused of killing the New Zealand-born photographer Trent Keegan was acquitted last month, after the prosecutors failed to make their case based on a lack of physical evidence, according to press reports.

On 28 May 2008, Keegan, then 33, was discovered, dead, in a drainage ditch in Nairobi, Kenya. Days before his death, he had told colleagues that he feared he would be murdered. However, despite the Kenyan police opening a homicide investigation, the murder remained unsolved.

But, in 2010, a documentary commissioned by TV One in New Zealand to look into Keegan's led to the arrest of Hesbon Amadade. The investigation was led by documentary-maker Ron Harley, who concluded that Keegan's killing was similar to the high-profile murder of a Kenyan Airways pilot in early 2008.

"It was just a lightning attack on somebody, beaten on the head, dumped, left for dead," Harley told TV One in May 2010. Amadade had been convicted of the pilot's murder and was already serving prison time when the TV One report was broadcast.

However, the court ruled that there was no physical evidence - in this case, Keegan's mobile phone - linking Amadade to the crime scene.

"The police presentation of the case was accurate, and all indications were that Amadade was involved," the Taranaki Daily News reports Nikki Keegan, the photographer's sister, as saying. "But without presentation into evidence of the cellphone she had to give him the benefit of the doubt and acquit."

Amadade will continue to serve his jail term over the murder of the Kenyan Airways pilot.

Originally from New Zealand, Keegan had based himself in the west of Ireland for three years before his trip to East Africa. He freelanced for several Irish newspapers and magazines, and had won several photography awards from the Irish Professional Photographers Association for his work. He carried out local commissions, but he also worked around the world.

In 2006, he served in Darfur with the international humanitarian organisation GOAL. He was joined on this assignment by freelance journalist Liam Horan, who would later receive his last photographs. That same year, six months after Hurricane Katrina hit Louisiana, he went to New Orleans, recording the plight of those still left homeless by the disaster.

"Trent was one of the most beautiful and genuine people you could ever meet," his sister, Nikki Keegan, told BJP in 2009. "All he ever wanted to do was help, so it is a huge sense of loss to us all that he has been taken from us at such a young age."

The police have always maintained that Keegan was the victim of a robbery gone wrong. Five men were first arrested in June 2008, and two of them were charged with the murder. However, they were later acquitted "for lack of evidence to link them to the robbery beyond reasonable doubt."

However, friends and family members believed Keegan was killed as a result of his work. Before his death, Keegan had been investigating the struggle over disputed land rights between a Maasai tribe in Tanzania and a Massachusetts-based company, Thomson Safaris. According to reports from friends, as well as documentation seen by BJP, a potential eyewitness, Nanyoi, a 24-year old tribesman, told Keegan that he had been shot by police after his tribe refused to comply with the company's demands to stop using the disputed territory for cattle grazing. Keegan had taken pictures of the injured Nanyoi, which BJP published on 23 July.

Keegan had emailed colleagues about his research and said that employees from the safari company had come to ask him questions about his report before he left for Nairobi.

Company officials have always denied any involvement with Keegan's death, or any wrong-doings regarding the local tribes. "We are a passive and peace-loving people, and in the irony of all ironies, pride ourselves on the strength of our relationships with our neighbours in Tanzania," said Judi Wineland, director of the safari firm.

Amadade's arrest, two years later, pointed that the police's theory might had been right. But, speaking to TV One, Harley said: "The only puzzling part is that the guy never touched his wallet, never touched his visa, never touched his passport, his cash, credit cards, and his boots, which are usually the first things taken in a mugging in Nairobi."