At least 21 people have been shot dead and seven wounded during a police raid on a Rio de Janeiro favela to capture the leaders of a drug-trafficking organization.

The deaths included a woman who was hit by a stray bullet in the exchange of gunfire between gang members and police in the Vila Cruzeiro favela.

The death toll puts the incident among Rio’s deadliest police operations in recent history. It comes one year after a raid of the Jacarezinho favela that left 28 people dead, prompting claims of abuse and summary executions. The episode sparked protests and also reignited debate over the proper use of police force in Rio, where a common local saying is, “A good criminal is a dead criminal.”

Just weeks after the Jacarezinho raid, a stray bullet hit a young pregnant woman during a police raid in another favela, and her death prompted further outrage.

Residents said on social media that heavy shooting began in darkness at 4am in a wooded area next to Vila Cruzeiro, sparking fear and panic in the slum. Gunfire began again in the afternoon, a Reuters photographer near the scene said.

Seven wounded people were taken to a hospital, including a policeman.

The police operation, backed by a helicopter, targeted leaders of Rio’s largest organized crime gang, the Comando Vermelho (Red Command), who, according to police intelligence, were hiding in Vila Cruzeiro.

“It was a very intense confrontation,” Colonel Ivan Blaz, spokesman for the militarized police force that led the operation, told reporters.

Outside a local hospital where victims were taken, residents of Vila Cruzeiro mourned the loss of family and friends and protested.

Rio state public prosecutors said in a statement they had opened a criminal investigation. They gave the military police 10 days to provide details about the operation, indicating which officials were responsible for each death and the justification for use of lethal force, the statement said.

The operation was conducted jointly by the military police and federal highway police. Police seized 16 vehicles, 13 automatic rifles as well as pistols and grenades, the police statement said.

The dead woman was identified as 41-year-old Gabriele Cunha, hit by a stray bullet even though she lived in a neighboring slum called Chatuba. Schools did not open for classes.

Earlier this year, Brazil’s supreme court established a series of conditions for police to conduct raids in Rio’s favelas as a means to reduce police killings and violations of human rights. The court ruled that lethal force should be used only in situations in which all other means have been exhausted and when necessary to protect life, and gave police 180 days to install devices to record audio and video on their uniforms and vehicles.