The authorities in Brazil say they are investigating reports that off-duty policemen killed nine people on Tuesday night in the northern city of Belem in revenge attacks.
The incidents happened hours after a policeman was shot dead by unknown gunmen outside his home.
Witnesses say men on motorbikes went on a six-hour killing spree, shooting nine people in several areas of Belem.
Six of the killings were carried out like executions, police said.
Tension began hours police officer Antonio Marcos da Silva Figueiredo, 43, was killed by three men .
Rumours of an alleged revenge attack spread quickly through social media overnight, causing panic among residents of the city, which is the capital of Para state.
Many preferred to stay at home, having received messages saying that several areas of the city would be “cleaned up”.
The authorities say “one line of investigation” is that the killings were an act of revenge by the dead policeman’s colleagues.
The official body responsible for reviewing police misconduct has launched an inquiry.
But the Para’s Security Secretary said many of the images being posted on social media had false information and used pictures of people killed in unrelated incidents.