Kigali said it was “shocked” by the news, with the Rwandan foreign ministry demanding answers from the Burundi authorities, amid worsening relations between the two neighbours.
UN experts have claimed that Kigali has recruited and trained refugees, including children, with the ultimate goal of removing Burundi President Pierre Nkurunziza from power, charges denied by Rwanda.
“Our citizen was being illegally detained, without judgement… We demand that Burundian authorities shed light on this sudden death, because it raises a lot of questions,” a Rwandan foreign ministry official said.
Burundi has been in political crisis since April 2015, when Nkurunziza announced a controversial bid for a third term in power.
More than 400 people have been killed in clashes since then and almost a quarter of a million people have fled the country, with the government blaming the killings on opposition forces.
“During the day he (Bihozagara) felt ill and was taken to the prison infirmary,” a detainee at Mpimba prison in the capital Bujumbura, reached by telephone, said on condition of anonymity. “Then they told us he had died 20 minutes later, shortly after 3pm (1300 GMT).
Detainees said Bihozagara had seemed in good health.
An official from Burundi’s penitentiary agency and the Rwandan foreign ministry official confirmed his death.
Bihozagara, a former youth minister in Rwanda as well as a former ambassador to Belgium and France, was arrested on December 4, 2015 by Burundi’s National Intelligence Service (SNR), an agency under the direct control of President Nkurunziza.
He was being held on suspicion of “working for the intelligence agency of his country”, an agent told AFP on the condition of anonymity.
Bihozagara had travelled regularly to Burundi since his retirement for business and to visit family.
Rwanda has denied the allegations levelled at it and insists the root causes of the crisis are inside Burundi’s borders.