(BBC) Police in Brazil have arrested nine of their fellow officers suspected of carrying out a series of killings on the outskirts of Sao Paulo.
Investigators say they were probably seeking revenge after the shooting of an off-duty policeman during a robbery in the area.
Nineteen people were killed within several hours on 13 August.
Witnesses in various locations said masked gunmen pulled up in a car before opening fire.
In many cases they checked the victims’ names before shooting or asked if they had criminal records.
The nine suspects were arrested in a major police operation across the Sao Paulo area.
More than 400 policemen took part in the operation. They seized guns, mobile phones and other objects.
The August attacks happened in the municipal areas of Osasco and Barueri.
Sao Paulo State Governor Geraldo Alckmin promised a swift investigation and set up a special task force.
But nearly two months after the attacks, only one suspect had been arrested.
Human rights groups say there is a high number of people are killed every year in Brazil either in police custody or by rogue policemen.
In the city of Rio de Janeiro alone military police have been responsible for more than 1,500 deaths in the last five years, the UK-based campaign group Amnesty International says.
Amnesty says that in some cases officers shot suspects who had surrendered or had been wounded.
Police trade unions say officers often come under fire from drug dealers and other criminals.