When São Paulo police found an estimated 50kg of cocaine paste hidden in boxes of soap powder that unwitting customers had bought in a neighbourhood supermarket, they were the first to crack a joke.

“Cleaning the streets of criminals,” they tweeted proudly.

Investigators were nonplussed. Detective Olívio Gomes Lira told the Guardian: “I had never found anything like this. It’s the first time I found cocaine inside a soap box.”

The drama began on Monday morning in the neighbourhood of Ermelino Matarazzo, on the eastern fringes of São Paulo’s urban sprawl, when a customer went to his local supermarket to buy a box of soap powder and found a tablet of pressed cocaine paste inside, the G1 news site reported. After complaining, he was threatened by gang members. A second man also bought a box of soap powder, and when he too found a tablet inside, he went to his local police station.

When police visited, the owner tried to flee in his 4x4 with the tablets of cocaine, said Lira, who estimated about 50kg of cocaine paste in tablets were found, which could have been transformed into as much as 200kg for sale.

A 49-year-old man was arrested. Three employees were also detained and police are investigating whether they too were involved.

“The first hypothesis we are working on is that someone put these on the shelf for sale by mistake,” Lira said.

Brazilian cocaine seizures between January and May soared 70% compared with last year, G1 reported, following a series of high-profile busts. Last month an airman was even found smuggling 39kg of the drug on a presidential plane that landed in Spain, part of president Jair Bolsonaro’s contingent as he headed to the G20 in Japan.

Much of the trade in São Paulo state is controlled by the PCC gang. “We believe he [the suspect] used the business as the front to wash dirty money,” he said, no pun intended.