Most details are secret, which may be the cause of conflicting reports:

Mexican and American authorities, for example, initially denied that the first fusion center, established over a year ago in Mexico City, shared and analyzed intelligence. Some officials now say that Mexican and American law enforcement agencies work together around the clock, while others characterize it more as an operational outpost staffed almost entirely by Americans.

Both countries appear to be flouting their own rules, or at least walking a fine line when it comes to extra-legal activity. Mexican law prohibits U.S. agents from conducting operations in Mexico, or from carrying weapons. And Mexico does have laws regarding wiretapping. All seem to be "flexible" when the war on drugs is involved:

Mexican and American officials say Mexico turns a blind eye to American wiretapping of the telephone lines of drug-trafficking suspects, and similarly to American law enforcement officials carrying weapons in violation of longstanding Mexican restrictions.