By Rachel Nuwer

 PUBLISHED AUGUST 5, 2016

Mr. K had never imagined it would be this bad. After leading a three-year undercover investigation of sea turtle smuggling in Vietnam, his home country, those efforts had finally yielded tips from informants, landing him on the doorstep of three warehouses in a rural area outside Nha Trang, a beach town in Khanh Hoa Province, about 250 miles northeast of Ho Chi Minh City.

As the police wrenched open the door, the first thing that struck Mr. K was the smell: an intense chemical reek, mingled with the stench of death. The second was the sight of the sea turtles themselves—thousands of those endangered animals, stacked one atop the other right up to the roof. Two of the three warehouses revealed such a scene.

“To see so many marine turtles, well, it was shocking. It was crazy,” Mr. K says. (He asked that his real name not be used because he continues to lead undercover investigations in Vietnam.)

A second raid the following month at a nearby farm uncovered even more dead turtles. The police eventually tallied some 7,000—the largest cache of marine turtles ever found.

That was in November 2014, and Vietnam has yet to arrest or prosecute anyone for the crime, despite the fact that Mr. K’s investigation revealed evidence that a wealthy local businessman is the criminal mastermind.

Almost all were either fully or partially taxidermied, bound for China to be sold as trophies.

As months have turned into years, and the case continues to stagnate, conservationists have come to see it as representative of Vietnam’s failure to tackle the illegal wildlife trade.

“You can nail the middle and low-level players all you want, but you’re not going to have an impact on the trade unless you go after the people who are behind it,” says Douglas Hendrie, technical adviser with the nonprofit group Education for Nature-Vietnam (ENV), which is based in Hanoi. ENV spearheaded the investigation and was Mr. K’s employer at the time.

“This case is our litmus test,” Hendrie says, “a way to say, OK, Vietnam, show us that you’re serious about dismantling this trade like you say you are.”

Vietnam is a major force in the illegal wildlife trade, serving as a supplier, consumer, and transit country for illicit animal products moving from greater Southeast Asia into China. But in early 2014 Prime Minister Nguyen Tan Dung publicly stated his intention to dismantle organized criminal trafficking of protected animals.

Later that year marine turtles joined tigers, elephants, bears, and pangolins on Vietnam’s list of fully protected species. Trafficking in any of those species is a criminal offense, entailing immediate prosecution and carrying a jail sentence of up to seven years.

 Mr. K got involved in the sea turtle case back in 2011 when a Vietnamese-registered fishing boat was seized in Philippine waters, and authorities found some 200 dead turtles on board. The following months saw several similar vessels turn up in Vietnam and the Philippines, all carrying dead turtles and registered to Sa Ky, a port in the central Vietnamese province of Quang Ngai.

So Mr. K headed to Quang Ngai, where he and ENV colleagues—posing as potential buyers, sellers, or students conducting research—asked around about sea turtles. They discovered a specialized group of divers who target marine turtles, and they met black market traders who buy those animals.