Police Commissioner Keabetswe Makgophe late Wednesday ruled out foul play in the death of Umbrella for Democratic Change (UDC) secretary general Gomolemo Motswaledi.
The popular politician died in a car crash on July 30, shortly after crossing the border from South Africa.
"The investigations reveal that Mr Motswaledi's death was the result of a road accident uninduced by any foul play," Makgophe said in a statement.
He said police felt the need to make the findings public, because of "speculations and innuendos" in mainstream and social media.
The opposition has alleged the government was involved in his death, claiming there was no evidence the car overturned or that the air bags were activated.
Duma Boko the president of the UDC -- a coalition of opposition parties -- said experts contracted by them, including pathologists, had not yet ruled out foul play.
Boko attributed the death of Motswaledi to blunt force trauma.
"There can be no doubt that this was no normal death in a car," adding that "it is fair that all concerned should be as thorough as Motswaledi would have been in everything he did for us".
The movement is poised to give the ruling Botswana Democratic Party led by President Ian Khama a serious challenge during the October 24 general elections.
Botswana is ranked among the most democratic countries on the continent, but the opposition complains it suffers harassment.