(CNN) A close aide to leading Rwandan opposition leader Victoire Ingabire has been found dead in the country, a party statement said.
Anselme Mutuyimana, 30, was found in the Gishwatu forest, in the northwest region of the country, by residents on Saturday morning.
Ingabire leads the FDU-Inkingi party, an unregistered coalition of opposition parties against President Paul Kagame, who has been in power since 2000
In a statement, the party said residents who found him said Mutuyimana appeared to have died by strangulation.
Placide Kayumba, a vice president for FDU who lives in exile in Belgium, told CNN that Mutuyimana was freed last summer from a six-year prison sentence for "political activism."
Mutuyimana had traveled from Kigali to visit his father and sister in the region but was killed before getting there, Kayumba said.
"The FDU-Inkingi condemns in the strongest terms the murder of Mr. Anselme Mutuyimana and call on the government to carry out a thorough investigation to identify the culprits to face the full force of the law," a statement from the party read.
Rwandan authorities say they have opened an investigation into the death.
"The investigations are still going on to establish the cause of his death and no suspect has been apprehended yet," Rwanda Investigation Bureau told CNN in an emailed statement.
Opposition politicians in Rwanda have often found themselves jailed on what they say are trumped-up charges for standing in opposition to Kagame.
FDU leader Ingabire returned to Rwanda to contest in the 2010 presidential elections after years of living abroad but was barred from running, and served eight years of a 15-year prison sentence on charges that included collaborating with a terrorist organization and "genocide ideology."
Ingabire was one of the more than 2,000 prisoners granted a presidential pardon last September.
Rwandan Justice Minister Johnston Businge said at the time her imprisonment and release were not political. "There is nothing political about her release; there is nothing political about her imprisonment," Businge said.
Diane Rwigara, a 37-year-old human rights activist, unaffiliated with any party, once hoped to run for president before being disqualified in 2017.
She faced a 22-year prison sentence until she was acquitted of charges of fraud and inciting insurrection by Kigali's High Court last year.
Boniface Twagirimana, another senior FDU member, has been missing since last October after he disappeared from a high-security prison where he had been held for five days following a transfer from another facility.
Rwanda's correctional service said he escaped from the prison, but party officials said they suspect "foul play."
Andre Kagwa Rwisereka, another opposition politician, was found dead with his head nearly severed just a month before the 2010 presidential vote.
Rwisereka's Green Democratic Party had been unable to secure registration to contest in the polls.