Kenyan authorities have suspended three policemen after a newspaper published a picture of an officer apparently shooting a man at point-blank range.
Three men are said to have been shot dead in the incident, which took place in central Nairobi.
The Daily Nation said the officers ordered the suspects out of a car, forced them to lie down and shot them.
The Kenyan police force has often been accused of carrying out extra-judicial killings.
The country's internal security minister said he had ordered the police chief to suspend the accused officers "with immediate effect" to allow a full investigation to tale place.
The Daily Nation said that a witness to the killings, which happened on Wednesday on one of the main roads through the city, had taken the pictures.
The apparent killing has been condemned by human rights campaign group Amnesty, which said such cases were all too common in the country.
"Eyewitness reports of this incident depict a disturbing image of police officers who are accustomed to acting with complete impunity. These appear to be blatant and deliberate killings that amount to extrajudicial executions," said Justus Nyang'aya.
The BBC's Will Ross, in Nairobi, says that the Kenyan police force is frequently accused of carrying out executions but proper investigations are so rare that Kenyans are left feeling the police are above the law.
In 2009 a UN investigation concluded that in Kenya, executions by the police were systematic and widespread.
After the country's disputed election three years ago, more than 1,200 people were killed, several hundred of them shot by the police.
The head of the force at the time, Major General Hussein Ali, is one of six men accused by the International Criminal Court of bearing the most responsibility for the post-election violence.