[1] In the late 1990s, Osiel Cárdenas Guillén, the former leader of the Gulf cartel, had other similar groups besides Los Zetas established in several cities in Tamaulipas.

[2] Each of these groups were identified by their radio codes: the Rojos were based in Reynosa; the Metros were headquartered in Matamoros; and the Lobos were established in Laredo.

[2] The infighting between the Metros and the Rojos of the Gulf cartel began in 2010, when Juan Mejía González, nicknamed El R-1, was overlooked as the candidate of the regional boss of Reynosa and was sent to the "Frontera Chica," an area that encompasses Miguel Alemán, Camargo and Ciudad Mier – directly across the U.S.–Mexico border from Starr County, Texas.

[2] Unconfirmed information released by The Monitor indicated that two leaders of the Rojos, Mejía González and Rafael Cárdenas Vela, teamed up to kill Flores Borrego.

[2] Other sources indicate that the infighting could have been caused by the suspicions that the Rojos were "too soft" on the Gulf cartel's bitter enemy, Los Zetas.