BBC News, Kigali
The detention of Rwanda's former spy chief is a cause for concern, a US-based human rights group says.
Once one of the president's closest confidents, Col Patrick Karegeya was demoted from his position last year and taken into military custody in April.
It is not known where he is detained and he has not been given access to a lawyer, Human Rights Watch (HRW) says.
The organisation says military authorities suggested his detention was for political reasons.
The army has declined to comment on these claims.
Fall from grace
It is a highly sensitive case for the Rwandan government.
As head of the country's external security service, Col Karegeya held a key position during Rwanda's occupation of neighbouring Democratic Republic of Congo.
Last year Col Karegeya was moved from the position of spy chief to army spokesman, widely seen as a steep demotion.
His arrest at his home in the capital, Kigali, in April underlined a more serious fall from grace.
The army say it is a case of indiscipline. The chief of general staff told the local press the colonel had persistently breached the army's disciplinary code of conduct and would therefore face charges.
But Alison Des Forges, a senior adviser to HRW, told the BBC that Col Karegeya has not benefited from due process under Rwandan law and the charges against him have not been made public.
In Kigali, speculation is rife about what lies behind the colonel's arrest.
It is widely seen as an indication of wider internal rivalries within the ruling Rwandan Patriotic Front (RPF), which keeps a tight grip on Rwanda.
No effective political opposition exists and within the RPF power is highly centralised around the president.
Recently, there have been reports of tensions. But these have dismissed by the party authorities.
HRW says the case of Col Karegeya highlights a continuing problem of lack of respect for due process in Rwanda's justice system.
Critics of the government have long claimed that the judiciary lacks autonomy and is used to suppress political opponents.