Zimbabwe's Prime Minister, Morgan Tsvangirai, left hospital yesterday as his party called for an independent inquiry into the fatal crash that killed his wife and left him with head injuries and chest pains.

US authorities have confirmed to officials from the Movement for Democratic Change that the truck that hit the premier's car, causing it to roll three times, was carrying Aids drugs donated by the American government. It was driven by a local contractor.

"There is no evidence at this stage of wrongdoing, but we are demanding an independent inquiry into the circumstances of the crash," a senior MDC member, Eddie Cross, told the Sunday Tribune. The party said it would accept an investigative team from neighbouring South Africa. However, a number of serious questions remain to be answered, amid continuing speculation in the troubled country that the prime minister, who joined his bitter political rival Robert Mugabe in an uneasy "unity" government last month, was the victim of an assassination attempt.

The 56 year old was travelling to his rural home south of Harare in a convoy of three vehicles late on Friday afternoon. His own security detail in the lead vehicle saw the USAID truck pass driving normally. However, it then veered into the oncoming lane, and would have hit Tsvangirai's vehicle head on had the prime minister's driver not taken evasive action. As it was the truck struck the rear of his vehicle, flipping it and killing Tsvangirai's wife Susan.

A local farmer contacted by MDC security reached the scene outside the town of Beatrice before the police and took photographs of the crash site. When police did arrive, they arrested him and confiscated still and video cameras. All three vehicles were later removed from the roadside, and the Zimbabwean truck driver was yesterday placed in "protective custody", further raising suspicions.

The crash comes after a fortnight in which there have been three separate incidents of senior MDC members or their allies being involved in suspicious car accidents. In each case, both front tyres of the new vehicles in which they were travelling blew out unexpectedly.

The presence of three members of the shadowy Joint Operations Command (JOC), who were somehow waiting at the Avenues Clinic in Harare before the prime minister had even arrived from the crash site, aroused further concern. Supposedly disbanded by the new unity government, the JOC was the body that led covert operations against the former opposition MDC, and made previous attempts on Tsvangirai's life. "How did they know he was coming in?" asked one MDC member, speaking on condition of anonymity.

The three men, Emmerson Mnangagwa, Patrick Chinamasa and the head of the feared intelligence agency, Happyton Bonyongwe were seen in the car park of the clinic and stayed there until Mugabe arrived with his wife to visit Tsvangirai nearly two hours later. The veteran president was then seen to confer with the trio before leaving.

The former trade union leader was said to be devastated at the loss of his wife. Friends described the couple who had been married for 31 years and had six children, as "soul mates", and said a difficult period of mourning awaited the prime minister. Should any evidence of foul play emerge, it would certainly unravel the already dysfunctional administration of political foes, still only three weeks old. Although Zimbabwe is beset by economic and health crises that the new unity government was meant to address, there has been almost no indication of progress.