Officials in Beijing have reportedly announced that “important progress” has been made in an investigation into anti-competitive behavior among chipmakers, including South Korean giants Samsung and SK Hynix, and US firm Micron.

“The anti-monopoly investigation into these three companies has made important progress…. [It] has yielded massive evidence,” Wu Zhenguo, head of China’s anti-monopoly regulator, was quoted by the Financial Times as saying.

Shares of Micron fell more than 6% on Monday.

Together, the three firms hold a virtual monopoly on the market for DRAM chips, widely used in consumer electronics. According to research firm IC Insights, DRAM prices jumped nearly 50% in 2017, after decades of decline.

“In the 34-year period from 1978-2012, the DRAM price-per-bit declined by an average annual rate of 33%. However, from 2012 through 2017, the average DRAM price-per-bit decline was only 3% per year.  Moreover, the 47% full-year 2017 jump in the price-per-bit of DRAM was the largest annual increase since 1978, surpassing the previous high of 45% registered 30 years ago in 1988!” the firm wrote in a report last March.

The Chinese investigation is not the first time the companies have faced antitrust action.

All three companies are currently contesting a class-action lawsuit filed in US courts, and Samsung and Hynix have both paid fines in the US and the European Union in the past for price-fixing.