According to the online edition of Wednesday's Wall Street Journal, the US sales unit of Samsung also has been subpoenaed.
Infineon Technologies in Munich was also contacted by the DOJ, but has not been subpoenaed, and will co-operate with the department, a spokesman said on Wednesday.
Micron said it intended to co-operate fully with the investigation.
"Micron does not believe it has violated US antitrust laws. The DRAM business is highly competitive and subject to extreme volatility. Competitive forces in today's market have led to DRAM prices reaching unprecedented lows," Kipp Bedard, vice-president of corporate affairs, said in a statement.
No one at Samsung's US office in California was available to comment. A DOJ spokeswoman confirmed that the department's antitrust division is conducting an industrywide investigation, but said that she could not comment on which companies are involved in the investigation.
The price of DRAM chips, commonly used as the main memory in desktop and notebook computers, is always changing and in 2000 began a gradual decline from around $9 (£6) per 128Mbyte chip to hit an all-time low of $1.50 (£1) per chip. At that price, manufacturers were losing money on each chip they sold.
However, the manufacturers stayed in the market to protect their share in anticipation of an upturn. The rise in prices began at the end of the year and, by April this year, the cost of a 128Mbyte chip was at around $5 (£3.36), but has recently fallen back to around $3.50 (£2.35).