An investigation into possible anticompetitive behaviour in the flat-panel display market widened today with four more big vendors saying they had been contacted by investigators.

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

The investigation came to light Monday when LG.Philips LCD Co. revealed that it had been subpoenaed by regulators in the US, 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, mobile phones and digital music players. The three companies being investigated are among the largest manufacturers of such displays.

The investigation comes on the heels of anticompetition probes in the DRAM (dynamic RAM) and SRAM (static RAM) markets. The DRAM investigation focussed on price fixing, which is when vendors cooperate to set prices artificially.

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 strongly committed to fair competition and ethical practices and forbids anti-competitive behaviour."

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 per cent on Tuesday, while Samsung's closed down two-thirds of a per cent. The general market, as measured by the Kospi index, fell 1 per cent. Sharp waited until after the market had closed to announce it had been contacted. Its shares rose 2 per cent on Tuesday.

In the DRAM investigation, Market leaders Samsung and Hynix Semiconductor filed guilty pleas 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 US Department of Justice (DOJ) although they did not disclose the nature of the DOJ's requests. The companies were Cypress Semiconductor and the US units of Mitsubishi Electric, Samsung Electronics, Sony and Toshiba.