MEXICO CITY — The biggest military corruption case here in recent years has worsened an already frayed relationship between American law enforcement officials and the Mexican Army, the institution most trusted and empowered by officials here to fight the drug war.

The case involves the arrests this month of four formerly high-ranking army officers, including a former under secretary of defense, who are suspected of passing information to the Beltrán Leyva drug gang for money. For some Americans, the arrests confirm a longstanding wariness of the army, and have reawakened concerns about how closely it may be linked to the gang, one of the top traffickers of cocaine to the United States and a particular focus of American drug agents.

American exasperation with the army reached a high point in 2009 when, fed up with what they saw as unusual foot-dragging by the army after it failed to act on American intelligence on the leader of Beltrán Leyva, the Americans turned to the Mexican Navy for help. The ensuing raid turned into a publicity coup for the navy when the gang leader was killed.

A meeting last year between American law enforcement agents and Mexican Army commanders to try to work through their differences ended abruptly. “It was basically 15 minutes, hello and goodbye,” said one official with knowledge of the meeting.