All four gorillas - three females and one male silverback - were shot but it was unclear who killed them or why, said the International Gorilla Conservation Program, a partnership of conservation groups that includes the World Wildlife Fund.
Residents heard gunshots Sunday night and alerted park rangers, who discovered the dead gorillas the following morning.
It is estimated that just over 700 mountain gorillas remain in the wild today. None exist in captivity.
"For such a small population, the unnecessary and indiscriminate killing of four mountain gorillas is a huge loss," the IGCP said in a statement.
The silverback was an alpha male. Alpha males are leaders within a group, in charge of leading the others to food and protecting them from danger. In his absence, the group is often at risk.
The animals were part of a 12-gorilla troop. Six others were confirmed safe, but a female and an infant are missing, the organization said.
The gorillas belonged to the Rugendo group living in an area often visited by tourists, providing valuable financial support to local communities.
Patrols were increased in that sector of the park with support from the Congolese army. Guard posts were being built to provide 24-hour surveillance of the park.
Earlier this year, two silverback males were shot dead in the same area of the park. It was believed they were killed by supporters of dissident warlord Laurent Nkunda. The skin of one of the gorillas was found at a latrine in a nearby rebel camp.
In May, a female gorilla was shot dead in the park. Her orphan baby is now being raised by the Institute for the Conservation of Nature in Goma.
"Seven gorillas killed in seven months is a horrifying statistic and a trend that cannot continue," WWF regional representative Dr. Kwame Koranteng said.
Mark Rose, chief executive of Fauna & Flora International, called the gorillas "helpless pawns in a feud between individuals."
The IGCP was formed in 1991 and is a partnership between the WWF, Fauna & Flora International, and the African Wildlife Foundation to conserve the threatened mountain gorillas and their forest habitat.
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