Police in Qinghai, northwest China, have busted the biggest pyramid scheme in the province's history, with seven people detained, local authorities said Tuesday.

More than 600 people invested in the scheme after being told that a company had filed an IPO in the U.S.. they were promised they would see a full return on their initial investment within two months, and their profits doubled within six months. The scam involved over 3 million yuan (483,600 U.S. dollars), according to the provincial department of public security.

China has toughened its stance against pyramid schemes. Last year, a five-month campaign led to the bust of 1,446 pyramid schemes, involving 3.7 billion yuan.