The brother of an exiled Rwandan general is reported to have disappeared after being arrested on Friday.
The wife of Lieutenant-Colonel Rugigana Ngabo told the BBC that she did not know where her husband was being held.
He is suspected of being involved in activities that threatened the country's security, an army spokesman told the BBC.
He is related to exiled General Faustin Nyamwasa, who served as chief of staff to President Kagame until 2001.
'Where could he be?'
Mr Rugigana Ngabo's wife, Peace, told the BBC: "They are telling me he is in Kigali and I've tried two prisons where he is supposed to be, and they are telling me he is not appearing on their list.
"Where could he be? My worst fear is, is he still existing ... where is he supposed to be?"
Rwandan army spokesman Jill Rutaremara told the BBC that Lt Col Rugigana's detention was not necessarily connected to his brother.
"It may, if the investigation finds so, but I am saying that someone is accountable for what he does and what he engages in but someone is not arrested because he has blood relations with somebody else," he said.
Mr Rugigana Ngabo's brother General Nyamwasa was wounded in South Africa in June in an apparent assassination attempt.
He served as chief of staff until 2001, and went into exile earlier this year.
The BBC's Martin Plaut says Rwandan politics is often less than transparent.
Human Rights Watch says a number of senior Rwandan officers have been arrested in recent months, but the reasons behind the detentions are often unclear.