* Reason for attack unknown
* Grenades easily available from past conflicts
By Kezio-Musoke David
KIGALI, July 13 (Reuters) - A grenade attack in a public market has wounded 21 people in western Rwanda, police said on Wednesday.
Police spokesman Theos Badege said four of those wounded on Tuesday evening in Rusizi district were in a serious condition.
"Right now we can't say the motive of the attack. We haven't arrested anyone as yet but security organs have intensified investigations," Badege said.
Rwanda's capital was hit by a string of grenade attacks last year, which the government blamed on two high-ranking army officers now in exile.
One of the those, Lieutenant-General Faustin Kayumba Nyamwasa, a former chief of staff and ambassador to India, denied the allegations last year saying the authorities had staged the attacks and then accused him of being behind them.
Nyamwasa now lives in South Africa. He was shot and wounded last June in an attack his wife blamed on Rwanda, a charge Kigali said was "preposterous".
Rwandan President Paul Kagame said last month the central African country was not under any security threat.
Six individuals suspected of being used by Nyamwasa as a conduit to finance activities intended to destabilise the country were arrested last month and are still being held.
Many grenades are left in Rwanda from lengthy conflicts in the Great Lakes region and are sometimes used to settle scores. Some 800,000 people were killed during 100 days of the 1994 Rwandan genocide. (Editing by Jon Hemming)
For more Reuters Africa coverage and to have your say on the top issues, visit; af.reuters.com/