David Salgado, alias "El Metro 4," was shot and killed on January 15 between the border cities of Matamoros and Reynosa, reported KNVO Noticias 48, who cite "extra-official" sources.

The death has yet to be confirmed by Mexico's authorities though intelligence sources in the country confirmed to InSight Crime that the Gulf Cartel leader was indeed killed and may have been betrayed by his own men.

Borderland Beat reports that there are two theories circulating as to who may have carried out the hit: one, it was the Gulf Cartel's former armed wing, the Zetas, who had entered Reynosa -- a Gulf Cartel stronghold -- sparking clashes between the two gangs; or, two, Metro 4's murder was the result of infighting within the Gulf Cartel.

InSight Crime Analysis

The once mighty Gulf Cartel has declined significantly in recent years, losing ground to rival organizations along with a number of key leaders. Most recently, the group's head, Jorge Eduardo Costilla Sanchez, alias "El Coss," was captured in Septmeber 2012, raising questions over who was next in line to run the organization.

El Metro 4 is believed to have been among the cartel's main leaders, and was a major figure in the Reynosa area. His death could serve to weaken the group further. 

If the Zetas were indeed behind the attack, it would not be the first time they have moved to take Reynosa from their progenitors. In November, for example, gun battles between rival gangs left 9 dead, in what may have been an attempted incursion by the Zetas into Gulf territory. This incursion may, in part, be related to the Gulf's incursion into Monterrey in recent months, which has left the Zetas reeling in that area.

If, on the other hand, it was Metro 4's own men serving him up to assassins, the question is to whom. Have members of the Gulf Cartel moved over to the Zetas? Or, is this an internal power play? The latter explanation would not be surprising as the Gulf Cartel has a history of internal warring, particularly as its national influence has declined.