SAN FRANCISCO  Counterfeit products are a routine threat for the electronics industry. However, the more sinister specter of an electronic Trojan horse, lurking in the circuitry of a computer or a network router and allowing attackers clandestine access or control, was raised again recently by the F.B.I. and the Pentagon.

The new law enforcement and national security concerns were prompted by Operation Cisco Raider, which has led to 15 criminal cases involving counterfeit products bought in part by military agencies, military contractors and electric power companies in the United States. Over the two-year operation, 36 search warrants have been executed, resulting in the discovery of 3,500 counterfeit Cisco network components with an estimated retail value of more than $3.5 million, the F.B.I. said in a statement.

The F.B.I. is still not certain whether the ring’s actions were for profit or part of a state-sponsored intelligence effort. The potential threat, according to the F.B.I. agents who gave a briefing at the Office of Management and Budget on Jan. 11, includes the remote jamming of supposedly secure computer networks and gaining access to supposedly highly secure systems. Contents of the briefing were contained in a PowerPoint presentation leaked to a Web site, Above Top Secret.

A Cisco spokesman said that the company had investigated the counterfeit gear seized by law enforcement agencies and had not found any secret back door.