Nine police officers were recently arrested on suspicion of participating in a killing spree in São Paulo, raising concerns over a new cycle of police-criminal retribution killings in Brazil.

The arrests occurred on October 8 in an operation involving over 400 police officers across São Paulo, reported the BBC. The detained officers are suspected of taking part in a shooting spree that left 19 people dead on the outskirts of São Paulo in August.

The shootings took place over several hours in at least 10 different locations, according to local media. Witnesses say the killings were perpetrated by masked gunmen traveling via car, in some cases checking the names of the victims and whether they had criminal records before opening fire.

Authorities believe the killings were in retaliation for the murder of a military police officer in early August by criminals robbing a gas station.

According to Folhapress, authorities also have evidence linking the same group of arrested officers — which consists of members from both the military and civil police — to 13 other recent deaths, bringing the total number of murders the group is suspected of perpetrating to 32.

InSight Crime Analysis

This recent spate of murders raises the spectre of a new cycle of revenge killings between police and criminals, something previously witnessed in several major cities across Brazil. In the Amazon city of Manaus, police were suspected of taking part in a July killing spree that was also thought to be in response to the murder of a police officer. This increasing aggression between police and gangs has contributed to a pervasive culture of violence within the Brazilian security forces.

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Although police violence is a long-standing issue in Brazil, it appears to have become a greater threat to the country’s overall security situation in recent years. According to one non-governmental organization (NGO), killings by police rose 37 percent in 2014, even though total homicides went up by only five percent. Official statistics also show killings by São Paulo police in 2015 are at a 10-year high.

Military police in particular have often been the perpetrators of violence and extra-judicial killings. A 2014 report by a local NGO found military police to be responsible for 1,567 civilian deaths in 2013 alone, while another recent study concluded that military police in São Paulo have killed at least 10,000 people since 1995.