Most members of the Sensitive Investigative Unit receive mandatory training and vetting by the DEA. But several current and former DEA agents said despite that vetting, the unit has long had a poor record of keeping information out of the hands of criminals. Among the most glaring problems, they say, is that Mexico doesn’t allow the DEA to scrutinize the unit’s supervisors in the same way as it does the unit’s members. Two law enforcement officials close to the Zetas case said their own inquiries revealed that a supervisor in the SIU was responsible. Former senior members of the Mexican Federal Police who worked closely with the unit did not respond to multiple requests for interviews.