The Los Zetas cartel has supplanted the Sinaloa mob as Mexico's largest drug-trafficking organization in terms of geographic presence, security consulting firm Stratfor said in a report.
At the close of 2011, Los Zetas operated in 17 states, or more than half the country, while its rival had operations in 16 states, Stratfor said, citing a report by organized-crime prosecutors.
Unlike the Sinaloa cartel, which tends to use bribery to achieve its aims, the Zetas "prefer brutality ... intimidation and violence," according to report published Tuesday.
Stratfor noted that "with a leadership composed of former special operations soldiers, (Los Zetas) are quite effective in employing force and fear to achieve their objectives."
The Zetas, whose stronghold is northeastern Mexico, moved last year into the central state of Zacatecas and the northwestern state of Durango, "achieving a degree of control of the former and challenging the Sinaloa Federation in the latter."
They also began to establish control over the Pacific coast state of Colima and its coveted port of Manzanillo.
In contrast to the alliances Sinaloa forms with other cartels, Los Zetas' ties with other gangs tend to be "more fleeting," the report said.
Despite losing 17 of its cell leaders and heads of plazas (drug-smuggling corridors) in 2011 to death or arrest, Los Zetas remained powerful and continued to be the dominant force in the Yucatan Peninsula, Stratfor said.
The Sinaloa cartel, meanwhile, lost at least 10 plaza bosses or top lieutenants last year, although Stratfor said it is unclear how those setbacks affected the cartel's operations overall.
The Texas-based security consulting firm did note, however, that a government crackdown had affected the methamphetamine business of the Sinaloa mob, the dominant producer of that synthetic drug following the disintegration of the La Familia Michoacana crime syndicate in early 2011.
According to the report, drug-related homicides declined last year in some areas - notably in Ciudad Juarez, Mexico's murder capital - but rose in other places, including the cities of Veracruz, Monterrey, Matamoros and Durango.
Los Zetas used to serve as the armed wing of the Gulf mob, which the Stratfor report said has split into two factions and is weaker than before but "seems to have maintained control of its primary plazas ... into the United States."
The government said earlier this month that 12,903 people were killed in drug-related violence between January and September 2011 in Mexico, an increase of 11 percent from the same period in the prior year.
The drug war death toll stood at 47,515 from December 2006, when President Felipe Calderon took office and militarized the struggle against the country's heavily armed drug mobs, to Sept. 30, 2011.
The murder total has grown every year since Calderon was inaugurated.
Unofficial tallies published in December by independent daily La Jornada put the death toll from Mexico's drug war at more than 50,000.