Testifying in the case in October 2018, Colonel Bruwer expressed fear for the lives of those on his investigating team after supporters of Mr. Mabuza who were demanding his release assaulted the detective. Colonel Bruwer’s car was damaged as well, but it is unclear by whom. The prosecutor in the case was also threatened, the newspaper Lowvelder reported.

Mr. Mabuza was first charged on six counts related to rhino poaching and released on bail in July 2018. He was later arrested again and released again on bail in January 2019. His trial is set to continue later this year.

Colonel Bruwer’s passion for wildlife began in childhood. His father worked for many years at Kruger National Park, the site of frequent poaching incidents, said Kobus van der Walt, who worked closely with Colonel Bruwer and is a lawyer with Mpumalanga Province’s asset forfeiture unit.

Poaching investigations would often take Colonel Bruwer and Mr. van der Walt back into the park to assess crime scenes, the lawyer said in a telephone interview on Thursday. On these trips the detective would recognize “small animals, plants and — just by hearing — a bird,” Mr. van der Walt said.

Colonel Bruwer also stood out because of the meticulous case files that he submitted to prosecutors, Mr. van der Walt said.

“They were neatly typed, in a specific font to make them presentable and easy to read,” he said. “There were no gaps in his dockets.”

Demand for rhinoceros horns spiked in the 1970s and 1980s because of their use in traditional Asian medicines and their status as a symbol of wealth, and conservationists have since fought to protect the animals.