Chinese police in the northern city of Tianjin recently seized 600 computers reportedly being used to mine bitcoin, after the local power grid operator noticed an unusual energy consumption.

According to the South China Morning Post (SCMP), authorities also seized an additional eight high-power fans. The power company noticed that the mining operation was creating sudden increases in line loss, that got to 28 percent at the peak.

This, to the power company, showed something was up. Subsequently, authorities found electricity was essentially being stolen. SCMP’s report reads:

“An investigation found that the junction box of the suspected power user’s electricity meter had been short-circuited – a typical way to avoid billing.”

Per the report, it isn’t clear when the equipment was seized, but local police claim this is the largest power theft case in recent years. As a result, five people are currently under investigation, while one has been detained.

Thanks to its cheap electricity and hardware manufacture, China has been a superpower when it comes to cryptocurrency mining. In 2017, allegedly 50 to 70 percent of all bitcoin mining hashrate originated in the country.

Earlier this year the country’s central bank, the People’s Bank of China (PBoC), revealed intent to toughen regional regulatory oversight over energy use to curb the power supply some miners have access to. As a result of China’s crackdown on cryptocurrencies, it’s unclear how many miners have moved offshore, or how many simply shut down their operations.

The 600 computers were seized after Chinese police arrested 9 in an alleged $13 million cryptocurrency pyramid scheme, that involved the sale of a fraudulent cryptocurrency dubbed DBTC.