RIO DE JANEIRO—Five police officers and at least seven civilians are dead following a weekend of violence in Rio de Janeiro as the 2016 Olympic host city struggles with a deepening security crisis.

Four state military police were killed when their surveillance helicopter went down Saturday night during a security operation at the notorious City of God favela, where shootouts between authorities an armed drug traffickers raged throughout the day.

Eyewitness video showed the helicopter spinning out of control as it fell. Rio state civil police and the air force have launched investigations into the cause of the crash to determine if the aircraft was shot down by gang members. A fifth police officer was killed Saturday afternoon while on patrol in another Rio neighborhood.

City of God residents said Sunday that they found the bodies of seven youths who had earlier gone missing after hundreds of police descended on the working-class community in response to the crash, setting up checkpoints and making a number of arrests.

In a video, the father of one of the victims said his son’s body showed signs of having been shot at point-blank range. Homicide police said they are investigating the deaths.