It appears that a former Rwandan soldier has been taken into custody by South African police over the shooting of Kayumba Nyamwasa over the weekend. According to an article in the South Africa Times, the man detained was a former officer who had served under Kayumba.
Other sources within the South African government indicate that the man detained was Francis Gakwerere, the former head of Laurent Kabila's presidential guard in 1997, when Rwanda controlled parts of the security services in the capital. He is retired now, but continues to live in Kigali and is in the RPF. (The South African newspaper got it wrong: they said he was also in exile).
At the same time, Ugandan journalist Daniel Kalinaki saw Kayumba after he was shot. He gives an account of what happened here. According to him, the shooter was speaking in Swahili and was not known to Kayumba or his wife.
The man detained is obviously not the same person as the shooter, who was speaking in Swahili and was unknown to Kayumba and his wife. The police have not been able to find the shooter yet.
Other sources close to the South African police indicate they are making good progress in the case.