In addition to with the casualties, the guerrillas took 13 people hostage but eventually released them.
The 2-million-acre (79,000-hectare) park is a UN World Heritage site and is home to diverse habitats, including swamps, snowfields, and volcanoes. Its most famous inhabitants are its mountain gorillas.
Over a hundred rangers have been slain in recent years trying to protect wildlife there.
Johannes Refisch of the UN-led Great Apes Survival Project said some 350 gorillas remain in Virunga.
He said it is impossible to say what was behind the latest attack—or if the Mai Mai were ultimately responsible—because the park is home to a host of competing factions.
Among those factions: former Rwandan military officers implicated in the 1994 genocide, Mai Mai rebels, and smaller armed groups, all vying for power and resources.
"It's sometimes quite difficult to see what really triggered the violence," Refisch said. "The situation is very fragile."
The latest attack was especially troubling, because it was in the park's northern sector.
Previous attacks had been limited to the south, said Noelle Kumpel, program manager of the bush-meat and forest conservation program at the Zoological Society of London. (Bush meat is the flesh of wild animals.)
The northern sector has been "so much more secure than the southern sector," Kumpel said. "Things seemed to be getting better, and this is a bit of a setback.
"Things have gotten worse again."
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