Nine people were killed in the Brazilian city of Belém on Tuesday in a spree that Brazilian authorities are investigating as a possible case of vigilante justice by off-duty local police officers. The killings -- six of which were completed execution-style -- took place just hours after a police officer was shot dead near his home by unknown shooters, the BBC reported.

The poor city of 1.4 million, similar to many in Brazil, has long had a tense relationship with police, but simmering tensions erupted into violence Tuesday, and investigators are looking into claims that the shootings were carried out as revenge for the officer's death. “All of the murders are under investigation, and if there is any evidence that police officers were involved, they will be investigated and held responsible,” Walrimar Sousa, a spokesman for the state Civil Police in Belém, told the Wall Street Journal. “It is too early to say police were involved.”

Residents followed along on social media as the spree played out, posting the latest details and rumors and lobbing accusations that the police were behind the attacks using the hashtag #ChacinaEmBelem. Some posted details and photographs of dead bodies that authorities said were fake, according to the BBC.

The allegations of revenge came in response to the fact that the killings occurred after elite police squad officer Antonio Marco da Silva was killed by three men. Belém is a very violent city even by Brazilian standards, with 45 murders per 100,000 residents vs. 25 per 100,000 nationwide and about five per 100,000 in the U.S.

Witnesses say men on motorbikes carried out the shootings in several areas of Belém. One witness described a victim being forced from a car and executed. Another told local media that his son's body had dozens of bullet holes.