Stirton saw firsthand how the news of a gorilla's death affected the rangers, whose only job was to protect the endangered apes. "That's a huge deal for these guys," Stirton said. "Because their identities are super tied to these animals."
Stirton and his crew, along with a group of rangers, set out for where the murdered gorilla had been reported - walking five hours through dense jungle.
When they arrived, they found carnage. A total of seven gorillas had been slaughtered, including one pregnant female. Bullet holes riddled the body of Senkwekwe, the male silverback.
Solemnly, the rangers began to cut down small saplings to carry the gorilla's bodies.
What struck Stirton most, as the brokenhearted rangers prepared to carry the murdered apes to a special burial ground, was the absence of any talking at all. "This was all very silent," he said.
It took the rangers more than seven hours to carry the gorilla's bodies to their final resting place.
It was immediately clear that the gorillas hadn't been killed by poachers, because their bodies were intact, but rangers couldn't figure out why so many had been shot.
It wouldn't come out until later that the gorillas had been murdered by people involved in the illegal charcoal trade - who were after the valuable hardwood trees in Virunga's protected gorilla area.
"What happened to the gorillas was basically an intimidation factor," Stirton said. "There was no reason, it was completely illogical."
Stirton had to flee the day after photographing the gorilla funeral processional, when he heard that the Congolese Army was looking for him. But he never forgot what he saw that day in Virunga.
His photograph of Senkwekwe's body being carried by the rangers made waves around the world, and since it was taken in 2007, the gorilla population within the park has had notable growth thanks to a dedicated warden.
But Stirton, who visits Virunga about once a year, was quick to caution that the gorillas are still in much danger. "It's just such a small number," he said. "Such a delicate part of the world."