Mashagiro is accused of involvement in an attack last July that killed five members of the Rugendo gorilla family.
The animals' corpses were left where they could be easily found, suggesting that poachers looking to sell body parts were not involved.
After the killings, Mashagiro was transferred to the Kahuzi-Biega National Park farther south for unknown reasons.
Dan Bucknell, regional program manager at the London-based Gorilla Organization, said he had met with Mashagiro just weeks ago to talk about conservation in Kahuzi-Biega and was surprised by the allegations against him.
"Many people, myself included, have expressed surprise that Mashagiro has been arrested on suspicion of this, because he has, over the years, been a strong ally and worked hard for the conservation of the Virungas," Bucknell said.
"So it does come as a shock if he is involved in this way."
The gorilla deaths—along with widely printed photographs of their bodies being carried out of the forest—garnered worldwide sympathy for a region where human suffering has often been ignored.
Human-rights groups, for instance, estimate that some 40,000 people are killed each month as part of the ongoing conflict in eastern Congo.
Mashagiro's arrest comes as WildlifeDirect and its partners have launched a new effort to fight the charcoal traders.
Backed by WildlifeDirect and other international partners, ICCN employees have erected a roadblock along a road leading to Goma."
A team of ICCN rangers and military police have seized more than a hundred tonnes of charcoal in the last ten weeks—a fraction of the total amount harvested in Virunga but a symbolic victory, said Chris Ransom, projects coordinator for the Zoological Society of London's Bushmeat and Forests Conservation Programme.
The UN Development Programme has also been distributing seized charcoal for free in Goma, where hundreds of thousands of people displaced from fighting in eastern Congo have sought refuge.
Ransom called Mashagiro's arrest an important signal that Congolese authorities—working in what many consider to be one of the world's most corrupt nations—were willing to get tough.
His alleged involvement in the trade was emblematic of the massive corruption in eastern Congo, conservationists say.
"In the past people have felt that they can get away with anything and not be arrested for it, and now it's been shown that the ex-provincial director has been arrested and will be taken to court and tried," Ransom said.
"This is a massive step for the ICCN for someone inside to be taken to court, particularly someone at such a high level."
Still Under Threat
The World Conservation Union lists mountain gorillas as critically endangered, and more than a hundred rangers have been slain in recent years trying to protect wildlife in Virunga.
(Related news: "Rare Gorillas Helpless as Congo Rangers Flee Rebels" [September 5, 2007].)
The situation in the region known as the "gorilla sector" is still tense. Earlier this month, one of the many rebel groups operating in the region warned rangers with the ICCN not to enter.
"They are very aggressive against the rangers of ICCN and have threatened to execute any of us who return to the gorilla sector," one ranger told the BBC news organization.
Some conservationists fear that all the attention brought to the gorillas has only made them easier targets for rebels and traders who run a host of illegal businesses in Virunga.
"Of course there are also other issues: There are land disputes [and] there is illegal farming in the park, sometimes backed by people with a certain influence," said Johannes Refisch of the UN Environment Programme's Great Apes Survival Project.
"It's very clear [the gorilla killings were] a political message. It has nothing to do with the gorillas per se. They use them as a message—Look, don't interfere with our business, otherwise you'll get in trouble."
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