It was just after sunset in Altamira, a small town nestled on a curve of the Xingu river in the heart of the Amazon rainforest, when Luiz Alberto Araújo, the secretary for the environment on the city council, arrived home with his family.

Before he could get out of his car, two men on a motorcycle pulled up and the passenger shot seven bullets into the 54-year-old, who was still sitting in the driver’s seat.

The killer got off the bike, opened the car door and shot him twice more. Araújo slumped on to his wife, who was seated beside him. Neither she, nor his two stepchildren, were injured. No attempt was made to steal anything.

The killing, on Thursday 13 October, had all the hallmarks of the sort of assassination that is common in the lawless Pará state, in the eastern Amazon, where illegal logging, clandestine mining and modern slavery are rife.

More than 150 environmental activists have been killed in Brazil since 2012, with studies showing the country accounts for half the global toll of such murders.

Many of those killed, including the high-profile cases of Chico Mendes, Dorothy Stang and Zé Claudio Ribeiro da Silva, were campaigners. But Araújo was a government official, and advocates say his murder is a rare and worrying development.

“The killing of Luiz Alberto Araújo marks a new low in the war waged against environmentalists in the Brazilian Amazon,” said Billy Kyte, campaign leader at the NGO Global Witness. “It sends a message that no one is untouchable.

“The government must urgently protect activists under threat and hold to account those responsible for this killing spree. Until prosecutions are made and protection is guaranteed, this deadly spiral of violence will continue unabated.”

Araújo’s home, Altamira, is a municipality in Pará, one of the poorest states in Brazil, with a land area larger than Greece but a population of just 110,000.

His work was a cross-section of the region’s environmental woes, from the battle against deforestation – which has risen by 24% in Brazil, recent figures show – to the consequences of the Belo Monte hydroelectric dam, which has been built nearby.

Araújo, who colleagues described as studious, serious and highly competent, had become used to receiving death threats. He had moved from a similar role in São Félix do Xingu, a neighbouring municipality, for that reason, they said.

“Without doubt he was afraid,” said Marcelo Salazar, of the Socio-Environmental Institute in Altamira, who worked with him. “He never said anything but anyone who works with the environment in Amazon towns will have a little bit of fear.

“It was a professional killing, there was no fight,” he added. “You have to be very careful with this work combating deforestation – and who you denounce.”

Araújo’s department this year granted a licence to a vast gold mine, called Esperança IV, in Altamira. Last month federal inspectors shut it down and fined its operators 50m reais ($16m) for breaching restrictions which barred the mine from any deforestation. Mercury and other pollutants were also leaking into the river Curuá, poisoning the food chain of the Kayapó indigenous tribe, inspectors said.

Araújo also reported the operators of Belo Monte, the fourth largest hydroelectric dam in the world, to federal prosecutors after his team found masses of dead fish.

Norte Energia, the operator, was eventually fined 35m reais ($11m) for the death of 16.2 tons of fish during the filling process of its reservoir. “He was assiduous in passing us this information,” said Ubiratan Cazetta, a federal prosecutor in Pará.

Earlier this year, federal inspectors in Altamira also busted a deforestation operation that had used modern slaves to clear 112 sq miles of forest, although officials played down Araújo’s involvement in the investigation.

The civil police in Xingu are investigating his murder. The detective leading the inquiry, Vinicius Sousa, said closed-circuit television footage was being analysed and his family and friends interviewed. Araújo’s environmental work and the inspections carried out by his secretariat were being considered as a possible motivation for his killing, he added.