SAN JOSE, Calif. | Intel Corp. and its much smaller rival Advanced Micro Devices Inc. have been subpoenaed by the Federal Trade Commission about possible anticompetitive behavior in the microprocessor market, the companies said Friday.

The move by the FTC to escalate its probe to a formal investigation is the latest in a series of legal challenges facing Intel, the world's largest computer chip maker. Antitrust investigations of Intel have been launched in several countries, including the U.S., based on complaints by AMD of unfair business practices that have stunted its growth.

The dispute centers on incentives Intel offers computer makers for buying Intel chips -- and the fierce retaliation AMD says those companies are threatened with if they offer products using AMD's chips. AP