The political party of Zimbabwean Prime Minister Morgan Tsvangirai is calling for an investigation into the car crash that killed his wife.

"We cannot talk of foul play…until it has been proved what has really transpired," said Tendai Biti, Tsvangirai's No. 2 in the Movement for Democratic Change party and the country's new finance minister.

At a news conference, Biti said the crash "could have been avoided" if Tsvangirai had been given the kind of motorcade that usually travels with President Robert Mugabe.

"If there had been a police escort, what happened would not have happened. The authorities could have avoided this omission," Biti told reporters.

Tsvangirai was released from the hospital Saturday and boarded a flight to Botswana.Zimbabwean Prime Minister Morgan Tsvangirai leaves a hospital in the capital, Harare. (Tsvangirayi Mukwazhi/Associated Press)Tsvangirai wore a baseball hat over bandages wrapped around his head as he walked out of the Harare facility.

Dr. Douglas Gwatidzo, a hospital spokesman, said the prime minister had head injuries and chest pains but was stable.

State television showed pictures of Tsvangirai in a neck brace, which Gwatidzo said was being used to keep him comfortable.

Doctors said Tsvangirai, who turns 57 on Tuesday, also complained of chest pains after he arrived for treatment. Susan Tsvangirai, his wife of 31 years and mother of their six children, died shortly after reaching the clinic. She was 50.

A truck with a USAID insignia on it slammed into a car carrying the couple and two other people, causing the vehicle to roll three times.

The truck driver involved in the crash told police he had fallen asleep at the wheel, said Dennis Murira, a spokesperson for Tsvangirai. State television said the truck had swerved on an uneven stretch of road.

The driver is in police custody, according to the Reuters news agency.

The accident happened a month after Tsvangirai was sworn in as prime minister of a unity government after years in opposition.

The fledgling government is made up of political rivals who have made it publicly clear that they distrust and dislike each other.

Mugabe visits Tsvangirai in hospital

On Friday evening, Mugabe and his wife, Grace, visited Tsvangirai, staying at the hospital for about an hour.

Mugabe later sent a letter of condolence to reporters about what had happened to his long-time political rival.

"We were all celebrating this major development [forming the unity government] when tragedy struck. It's a sad occurrence indeed," he said.

A former trade union leader, Tsvangirai won the most votes in the first round of last March's presidential election, but he pulled out of a run-off with Mugabe because of violence against his supporters.

The Movement for Democratic Change and Mugabe's ZANU-PF party held months of negotiations before finalizing details of the new government, which is faced with a collapsed economy and a growing health crisis.

With files from the Associated Press