Esmond Martin, a U.S. investigator who spent years going undercover to expose the worldwide ivory trade from slaughter to marketplace, was found stabbed to death Sunday in his Nairobi home.

Police in Kenya haven't made an arrest or named a suspect, according to the Associated Press. They said it appears that Martin, 75, was the victim of a botched robbery, but the investigation is continuing.

Martin's wife, Chryssee Martin, became worried after Martin didn't respond to phone calls, and she went to check on him at their home in a suburb southwest of the city's business district. She found him in the afternoon, dead on the bed with a stab wound to the neck, authorities said.

He had recently returned from a trip to Burma, according to the BBC, and was writing up his findings about the ivory trade there.

It was work he'd been doing for decades: putting his life at risk to document the illegal sale of elephant ivory and rhino horn in Vietnam, Laos, China, African nations and the United States. He had been a U.N. special envoy for rhino conservation and had published reports funded by Save the Elephants.

He often posed as a buyer, haggling with vendors to try to ascertain market prices for ivory, rhino horns and the things made from them. He watched as Chinese mine workers in Africa purchased ivory chopsticks and carvers in other countries fashioned ornate sculptures out of tusks. He documented illegal actions involving ivory that had slipped through borders. Martin took detailed photos of ivory shops and illegal carvings, meticulously tracking the trade that has decimated populations of elephants and rhinos.

According to the AP, syndicates from China, Vietnam, South Korea and Thailand have long been involved in such trafficking.

As news of his death rocketed around the world, condolences poured in from others in similar conservation fights. Paula Kahumbu, the chief executive of WildlifeDirect and chairwoman of the National Museums of Kenya, tweeted that Martin was “a global authority” on the illicit trade.

“He was at the forefront of fighting #WildlifeCrime in [Kenya], exposing rhino horn & elephant ivory trade,” the World Wildlife Fund's office in Kenya tweeted. “He remains an inspiration for all of us.”

A  century ago, 5 million elephants roamed the plains and forests of Africa. Now fewer than 400,000 remain, devastated by poaching and the destruction of their natural habitats, the AP reported. Rhinos have had a similar fate. Fewer than 30,000 remain in the wild.

For Martin, it was no secret where all that ivory was going. But he and his collaborators provided something that could motivate governments, policymakers and concerned citizens to act: detailed evidence and a road map of the trade.

He told NPR in 2012 that he found nearly 4,000 illegal ivory pieces for sale in the southern Chinese city of Guangzhou. That's twice as many as he'd found in a similar survey a decade earlier.

A recent report about the ivory trade in Laos gave a glimpse into how meticulous Martin's ivory research was.

“We learned about where the ivory was being carved and processed and by whom,” he wrote in the report for Save the Elephants that was co-written by Lucy Vigne. “We visited towns and casino areas where we suspected there was trade in ivory to survey the retail outlets and ivory items on view for sale. …We learned about the ivory trade from carvers, middlemen and vendors in shops selling ivory.”

Martin, a New York native, had been involved in conservation and research for the past half a century. He first came to Kenya in the 1970s, according to the BBC, when large numbers of elephants were being killed for their tusks. For nearly two decades, he worked for Save the Elephants, which helps monitor the illegal killing of elephants. In recent years, he had increasingly focused on the demand side of the ivory and rhino horn market, hoping that decreasing the desire for the products would ultimately help save animals.

The efforts appeared to be working. Last year, China banned the ivory trade there, something The Washington Post's Simon Denyer said “has thrown a lifeline to African elephants and brought new hope in the battle to end the poaching of tens of thousands of animals every year for their tusks.”

The government closed nearly 200 ivory-carving workshops and retail outlets. “With the end of the legal ivory trade in China, the survival chances for elephants have distinctly improved,” Esmond told the Star, a Nairobi newspaper, last year.

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