Banners signed by the Gulf Cartel

Two large Mexican drug cartels -- once thought to be reeling due to infighting, pressure from authorities, and constant assaults by rivals -- appear to be on the rise again, according to law enforcement and independent crime analysts.

The Gulf Cartel and the Beltran Leyva Organization have surged in various parts of the country, including critical, strategic drug production and trafficking corridors in northeast and western Mexico.

After a prolonged battle, the Gulf Cartel has taken control of most of the industrial hub of Monterrey, which was under the Zetas' domination since until at least 2010, when the latter broke from the Gulf and the two began a pitched battle across the northeast of the country.

Mexican and foreign government intelligence analysts, as well as the independent crime-watching service Southern Pulse, say at least three-quarters of Greater Monterrey Metropolitan -- which includes the municipalities of Apodaca, Garcia, General Escobedo, Guadalupe, Juarez, Monterrey, San Nicolas de los Garza, San Pedro Garza Garcia, Santa Catarina, and Santiago -- is under Gulf control now.

This dramatic and somewhat surprising assessment comes after years of pinpoint assaults on Zetas' drug distribution points and safe houses by both the Gulf and their allies, the Sinaloa Cartel.

Monterrey is important for many reasons. The city lies along an important trafficking corridor leading to Nuevo Laredo, where more commercial vehicles cross into the United States than at any other point along the 3,169 km border.

The Zetas remain the power in Nuevo Laredo, and it is the de facto headquarters of the organization following the death Zetas leader Heriberto Lazcano , alias "Z-3," at the hands of Naval forces in October. The Zetas' new leader, Miguel Treviño, alias "Z-40," has long been based in the Nuevo Laredo area.

Monterrey also has numerous sources of local revenue, which the Zetas, more than many other large Mexican criminal groups, use to sustain their forces and expand into new territories. The plethora of money-making operations in the city include local drug peddling, kidnapping, extortion and theft.

(For a more complete accounting of the Zetas' history and revenue streams in Monterrey, see The Zetas and the Battle for Monterrey)

The steady erosion of the Zetas' stranglehold in the city has coincided with a drop in homicides. According to state government statistics, homicides in the municipality of Monterrey, for example, dropped to 11 in November, compared to 73 in August. (See statistics for all Greater Monterrey here)

The resurgence of the Gulf is even more surprising considering the recent arrest of the group's top leader Jorge Eduardo Costilla Sanchez , alias "El Coss." Security analysts consulted by InSight Crime speculated that this resurgence in Monterrey may also be related to the deterioration of the Zetas, who, in addition to losing their top leader, have suffered from infighting in recent months.

In this fratricidal struggle, real and symbolic figures make for legitimate targets. In January, 17 members of the Vallenato group Kombo Kolombia were intercepted and killed after playing at a private party. Vallenato, the Colombian music named for its birthplace Valledupar, is reportedly a favorite of the Zetas. The way the music group was tracked and massacred suggests the victims were more of a bloody message than an attack of any operational value.

Still, the situation in the northeast remains in flux. In January, InSight Crime, and other media, reported the death of David Salgado, alias "Metro 4," a top level Gulf member, in a firefight between the cities of Matamoros and Reynosa. Authorities did not confirm his death, but one intelligence official in the area told InSight Crime that Metro 4 was betrayed by his own men.

Meanwhile, the Beltran Leyva Organization (BLO), a Zetas' ally that was also thought to be in decline, has recovered and is threatening its former boss, the Sinaloa Cartel, in other parts of the country. The latest blow came with the reported death of Felix Adolfo Jauregui Meza, alias "El Paletero," a Sinaloa Cartel lieutenant in Sonora where the BLO and Zetas have been steadily reclaiming territory.

Coupled with repeated blows the BLO has delivered to the Sinaloa group in their namesake and the emergence of a new strongman tied to their loose, nationwide alliance with the Zetas and the Juarez Cartel, it appears the BLO has returned to the form that once made it one of the most feared cartels in Mexico.

In January, the US government said Fausto Isidro Meza Flores, alias "Chapito Isidro" -- a longtime BLO ally who hails from the same area as the Beltran Leyva brothers, as well as Sinaloa Cartel heads, Joaquin Guzman, alias "El Chapo," and Ismael Zambada, alias "El Mayo" -- plays "a significant role in international narcotics trafficking." 

Chapito Isidro has a strong presence in Sinaloa and Sonora, two longtime BLO areas of operation. To be sure, Isidro reportedly organized a brutal 2010 ambush in Sonora of Chapo Guzman's men, which left some 29 dead and many others wounded.

The BLO seems to be winning legal struggles as well. The case against five high-level retired military officials, who were allegedly part of their structure, is falling apart, the Attorney General's office told a judge

Perhaps more importantly, one of their top leaders, the infamous Alfredo Beltran Leyva, alias "El Mochomo," has not been extradited to the United States and by some accounts continues to operate the organization from prison.