There has as yet been no suggestion of foul play over the crash
Early reports from an investigation by Zimbabwe's MDC suggest that a car crash which killed PM Morgan Tsvangirai's wife may not have been accidental.
Senior MDC officials told the BBC that investigators were questioning the cause of the March crash, in which Mr Tsvangirai was injured.
At first, Mr Tsvangirai said he thought the crash had been an accident.
The MDC, formerly in opposition, entered a power-sharing agreement with President Robert Mugabe in February.
But the evidence which has so far come to light in the MDC investigation leads senior MDC officials to believe that the whole affair was highly questionable, says the BBC's John Simpson, who has been in Zimbabwe.
The car carrying Mr Tsvangirai and his wife was hit by a lorry on a main road south of Harare.
US embassy officials say the lorry belonged to a partner organisation of the US government aid agency USAID.
Mr Tsvangirai's car was accompanied by two cars provided by the government, and driven by men from Zimbabwe's Central Intelligence Organisation, which is loyal to President Mugabe. One car led the way, the other followed Mr Tsvangirai.
Morgan and Susan Tsvangirai had been married for 31 years
The MDC sources say the lorry involved in the crash sideswiped Mr Tsvangirai's car, hitting the rear offside.
The car veered across the road and landed in the bush on the far side, upside down. Mrs Tsvangirai was thrown out and killed, and her husband was injured.
The MDC stands to gain hugely in political terms, our correspondent says, if it can prove that it was all a deliberate attempt to kill Mr Tsvangirai soon after he had entered the power-sharing agreement, something which Mr Mugabe's Zanu-PF party seems to regard as a potential threat to its position.
But the MDC officials pointed out that the apparently deliberate deaths of senior politicians who have been seen as a threat to the Zanu-PF leadership are nothing new.
At least two have died in highly suspicious road accidents in the past.