Exiled former Rwandan intelligence chief Patrick Karegeya has been apparently murdered in a Johannesburg hotel room, South African police say.
They say the dissident might have been strangled, with a rope and bloodied towel found in the hotel room safe.
Mr Karegeya was stripped of the rank of colonel after falling out with his former ally, President Paul Kagame.
President Kagame’s allies have previously denied accusations of links to a series of dissident attacks.
Mr Karegeya, 53, formerly head of Rwanda’s foreign intelligence service, had lived for the past six years in South Africa, where he had been granted political asylum.
A fellow exiled dissident, former army chief Faustin Kayumba Nyamwasa, has survived two assassination attempts since fleeing to South Africa in 2010.
The pair formed a new opposition party – the Rwanda National Congress – in 2010.
Gen Nyamwasa told the BBC that Mr Karegeya had gone to the upmarket Michelangelo Towers hotel to meet “somebody he knew very well, somebody who had come from Kigali”.
He accused the Rwandan government of being behind the killing.
Rwanda’s ambassador to South Africa, Vincent Karega, dismissed this as an “emotional reaction and opportunistic way of playing politics”, reports The Associated Press news agency.
“We encourage the authorities to really look into the matter so that we know exactly what happened,” the Reuters news agency quotes him as telling local radio.
A police statement on Mr Karegeya’s death said: “Preliminary investigations revealed that his neck [was] swollen – there is a possibility that he might have been strangled.”
He leaves a widow and three children.
Rwandan exiles in several Western countries including the UK and US say local security agents have warned them of plots to kill them.
The Rwandan government has denied trying to kill its opponents.
Mr Karegeya and Gen Nyamwasa were among four exiled former top officials for whom Rwanda issued international arrest warrants in 2011.
A military court earlier sentenced them to long jail terms in absentia for threatening state security and promoting ethnic divisions.
Both men were part of Mr Kagame’s rebel forces which came to power in 1994, ending the genocide of their fellow ethnic Tutsis.
Mr Kagame has been accused of not tolerating opposition.
He maintains that Rwanda needs a strong government to prevent a return to ethnic conflict.
Critics of Rwandan President Paul Kagame tend to flee the country as soon as they fall out with him because they fear it is too dangerous to stay.
But many have met mysterious deaths abroad, although the president and his allies have always denied any responsibility.
Rwanda’s first post-genocide Interior Minister, Seth Sendashonga, was shot dead in Nairobi shortly after resigning in 1996, leading to a diplomatic row between Kenya and Rwanda. The Metropolitan Police has warned two dissidents based in London of threats to kill them. And there were two attempts to kill former army chief Faustin Kayumba Nyamwasa in South Africa.
The apparent murder of Patrick Karegeya, also in South Africa, will make Rwandan dissidents feel even less safe. His death is a big blow to the opposition party he founded, the Rwanda National Congress. But it also has the potential to be a huge embarrassment for President Kagame.
Ally Yusufu Mugenzi BBC Great Lakes editor