Brazilian police in the city of Manaus are investigating a surge in murders after 35 people were reported to have been killed over the weekend.
The shootings, in the heart of the Amazon, began after a police sergeant was shot dead outside a bank.
Officials are investigating whether they are part of a drug gang war or the result of police officers avenging the death of their colleague.
There are normally between two and three murders per day in the city.
The nature of the shootings has raised suspicions of police involvement.
Several of the murders happened at almost exactly the same time in different parts of the city, suggesting coordination between the killers.
The shootings were reportedly carried out in an execution style.
Local officials say that another possible explanation could be local drug gangs settling scores.
Manaus is the capital of Amazonas state in the north of Brazil.
It was of the host cities for the 2014 World Cup. England lost 2-1 to Italy in their opening game of the tournament in the Arena da Amazonia.
The stadium will host will some of the football matches during Rio’s 2016 Olympic Games.
Gun crime in the country is at its highest in 35 years, according to a recent UN report.
Using the latest data available, it found that 42,000 people were shot dead in 2012.