SAO PAULO (AA): Gunmen in the Brazilian city of Sao Paulo killed 19 people and injured seven others in a series of attacks on Thursday night, officials said.

“We are not ruling out any motive at the current time,” Sao Paulo Secretary of Public Security Alexandre de Moraes told a news conference on Friday, giving repeated assurances that the “deadliest massacre of the year” would be thoroughly investigated.

He urged caution over suggestions that the murders – in the Osasco, Barueri and Itapevi neighborhoods – were revenge attacks by police officers.

Local media reported the hooded gunmen had asked people about their criminal backgrounds, with those admitting past criminal activities being shot.

Osasco Mayor Jorge Lapa told the G1 news website he believed the attacks could be related to the deaths last week of two police officers.

Local reports also suggested that some of the weapons used in the attacks, which took place over a three-hour period, were restricted to the military.

Police investigation teams are still working at around 10 crime scenes. One officer quoted by the Folha de S.Paulo newspaper said he had “never seen a night with so many deaths”.

Police have yet to confirm any arrests in connection to the attacks, which began at 8.30 p.m. local time (2330GMT).

Authorities had recently celebrated a reduction in murders across the city, with de Moraes credited for the fall in crime since his appointment at the end of 2014.

The Barueri area, for example, had seen a 36.4 percent drop in the number of homicides in the first half of 2015, compared to the same period in 2014, according to Folha de S.Paulo.

Commentators said the killings would serve as a “reality check” for the city, where crime remains a major concern.

Brazil is thought to remain the world leader for murder. In 2012, the most recent year with comprehensive data, the Brazil government says it registered 47,000 murders although the World Health Organization estimated the figure as 64,000.