Four men were shot dead in 2006, just days after police launched a "shoot-to-kill" policy in an attempt to control an upsurge of armed robberies.

The police originally said the four were suspected armed robbers.

But an official investigation found three of the men were gemstone dealers and the other was a taxi driver.

After a lengthy legal process, Judge Salum Massati ruled that the prosecution had failed to prove its case beyond reasonable doubt.

Reporter in Dar es Salaam, says the case has been attracting huge publicity, with Tanzanians divided between those who believed the officers were guilty and others who say the evidence was too thin.

The accused officers included Abdallah Zombe, the former regional police chief of Dar es Salaam.

At the time of the shooting, Mr Zombe had told the BBC: "We kill notorious robbers, not innocent people."

The police accused the men of robbing a jewellery shop and engaging them in a shoot-out.

But after an outcry from the public and the relatives of the dead, an official inquiry concluded there had been no shoot-out and the four men had not been criminals.