However, Phoenix Police have not confirmed the men were Mexican Army members

Conscription

The military draft lottery

Each year, the SEDENA (Secretary of National Defense) requires every eighteen-year-old man and every man who be that age within twelve (12) months of the draft date of a designated Clase de Servicio Militar Nacional (National Military Service Class) to report to his municipality's designated military recruitment center. There, he will register to the conscription program with his birth certificate and standard-size, head-and-shoulders portrait photographs in order to be issued a precartilla (pre-military identity card) with a serial number, photograph, right thumb fingerprint, weapons licence number, and personal data (address, current schooling level, etc.) that finally, after the draft lottery and a year of service, earns him full conscript status and a Cartilla de Servicio Militar Nacional (Military National Service Identity Card), informally, cartilla (military I.D. card) attesting to his having fulfilled his civic obligation in duty to the nation. The SEDENA's lottery determines who will be exempted from or drafted for military duty — either with the army or with the navy.

Formerly, the military service lottery required the presence of every man of a National Military Service Class; he stood at attention, awaiting either conscription or exemption; each man's name was called aloud and a child drew a coloured ball from a bag — the colour determined the man's conscription to or exemption from military service. Today, military service status selection is computerised, but the results are announced as before: White ball (Army service), Black ball (exemption), and Blue ball (Navy-Marines service). Currently, a seal is stamped to the "precartilla" identifying the bearer's military draft lottery status.

The Cartilla

The drafted men attend and participate in week-end military training comprising basic military training with weapons, and combat tactics, yet it emphasises education, history, physical fitness, and military discipline for one complete year. Afterwards, the precartilla (pre-military identity card) is returned to the conscript with an added page certifying his status as having fulfilled his national military service, identifies the military branch, the unit, rank, etc. The document then acquires full status as the Cartilla del Servicio Militar Nacional (Military National Service Identity Card), informally Cartilla; this status is recorded to the National Defense Secretariat files.

This document (Military National Service Identity Card) is an important form of Mexican national identification, and its existence is almost always requested by private and public employers, however, this identity document has ceased being required for obtaining a passport for international travel.