An investigation into possible anticompetitive behavior in the flat-panel display market widened Tuesday, with at least four more big vendors saying they had been contacted by investigators.

Samsung Electronics said it had been subpoenaed by regulators in the U.S., South Korea, and Japan, while Sharp and Taiwan's AU Optronics were contacted by the Japan Fair Trade Commission and the U.S. Department of Justice (DOJ). A U.S. subsidiary of Taiwan's Chi Mei Optoelectronics was also contacted by the DOJ.

The investigation first came to light Monday when LG.Philips LCD Co. revealed that it had been subpoenaed by regulators in the U.S., South Korea and Japan.

The probe centers on TFT (thin-film transistor) LCDs, according to Samsung. They are used in a wide range of electronics products including flat-panel televisions and computer monitors, laptop computers, cell phones and digital music players. The companies being investigated are among the largest manufacturers of the displays.

The investigations center on price-fixing, according to the European Commission, which confirmed Tuesday that it too has opened an investigation. Price-fixing is when vendors cooperate to set prices for their products artificially.

The Commission is seeking to "ascertain whether there is evidence of a cartel agreement and related practices concerning price fixing," it said in a statement. It sent formal requests for information last Friday to an undisclosed number of firms that sell TFT LCDs, it said.

The investigation comes on the heels of anticompetition probes in the DRAM (dynamic RAM) and SRAM (static RAM) chip markets. The DRAM investigation also focussed on price-fixing.

Sharp spokeswoman Miyuki Nakayama said the company was surprised to have received the summons. Sharp has a policy of "fair and ethical management," she said. Cho Sung In, a spokeswoman for Samsung in Seoul, said Samsung Electronics is committed to "fair competition and ethical practices."

Among other LCD manufacturers, Sony said its joint-venture with Samsung, S-LCD, had not been contacted by the investigators.

Shares in LG.Philips dropped 4 percent Tuesday, while Samsung's closed down two-thirds of a percent. The general market, as measured by the Kospi index, fell 1 percent. Sharp waited until after the market had closed to announce it had been contacted. Its shares rose 2 percent Tuesday.

In the DRAM investigation, Market leaders Samsung and Hynix Semiconductor pled guilty in 2005 to price-fixing and received fines of $300 million and $185 million, respectively. Japan's Elpida Memory had to pay $84 million and Germany's Infineon Technologies paid a $160 million fine.

In October several manufacturers of SRAM chips said they had been subpoenaed by the U.S. Department of Justice (DOJ) although they did not disclose the nature of the DOJ's requests. The companies were Cypress Semiconductor and the U.S. units of Mitsubishi Electric, Samsung Electronics, Sony, and Toshiba.

(Paul Meller in Brussels contributed to this report.)