(CNN) -- A human rights group Wednesday urged the Rwandan government to let independent experts conduct an autopsy on an opposition leader killed weeks before the election.
Andre Kagwa Rwisereka, vice president of the opposition Democratic Green Party, was found dead after he went missing on his way to a meeting in the capital, Kigali.
His body was found in a wetland area, with the head nearly severed, according to party president Frank Habineza.
Habineza said he and Rwisereka, whose service was held Tuesday, had received death threats since February.
"We are very threatened as a party and as individuals," he said. "We are scared. We are also sad about the demise of our colleague."
Human Rights Watch said police gave different explanations on the death.
At first, authorities said he was the victim of a robbery, Human Rights Watch said. The rights group said its investigation revealed that the victim did not have any valuables on him when he was attacked.
Police later said he was killed in a financial dispute with a man who is in custody and remains a prime suspect, Human Rights Watch said.
The conflicting statements have sparked doubt and confusion, and an independent probe would confirm or dispel the different explanations, the rights group said.
"This is the second killing of an outspoken critic of the Rwandan government in less than a month," said Kenneth Roth, executive director of Human Rights Watch. "An independent autopsy and inquiry are necessary to determine what happened to Rwisereka."
Last month, Jean-Leonard Rugambage, a journalist with a local independent newspaper, was shot dead outside his home in the capital.
Earlier this year, the nation's president, Paul Kagame, hit back at critics who said opposition activists are facing increasing threats, attacks and harassment ahead of th election.
"You tend to make a judgment of a country, 11 million people, on what a couple of people have said and (they) don't take into account what Rwandans say," Kagame told CNN.
"Nobody has asked the Rwandans ... it's as if they don't matter in the eyes of the human rights people. It's our own decisions in the end."
Kagame said everyone in Rwanda has to play by the rules and be accountable.
"There has to be leadership to make things move in the right direction," the president said.
Rwanda is scheduled to hold elections next month.