Mexico's military discovered over 1.5 million liters of stolen oil in Veracruz state, a region dominated by the Zetas, pointing to the growing importance of oil theft to the country's biggest criminal gangs.

Soldiers made the discovery after acting on information provided by citizens in the municipality of Minatitlan, southeast Veracruz, the Defense Department told EFE. Along with the oil -- over 90 percent of which was still in crude form -- two tanker truckers and eight tanker trailers were seized from two clandestine depots.

The depots were found close to a refinery run by state oil company Pemex.

Mexico's Attorney General's Office announced earlier this month that it had opened several preliminary investigations into companies accused of buying fuel stolen from Pemex. Pemex is reportedly taking legal action against 14 US companies alleged to have bought fuel stolen by the Zetas gang.

InSight Crime Analysis

During President Felipe Calderon's time in office, large-scale criminal groups have become increasingly involved in oil theft in Mexico. Once the reserve of small-time gangs, in recent years there has been evidence of both the Zetas and their rivals the Sinaloa Cartel gaining a growing stake in the trade.

In the same period, the amount of fuel stolen from Pemex has risen considerably. Some 3 million barrels (just under 477 million liters) of oil were stolen in 2011, a jump of 52 percent on the previous year, and representing a loss of $475 million to the company.

Pemex employees and Mexican government officials are sometimes complicit in oil theft, as the Senate has noted.

Though the two illegal depots were discovered in Veracruz, a Zetas stronghold, the state worst afflicted by oil theft is now Sinaloa, which overtook Veracruz and Nuevo Leon last year in the number of illegal siphons detected, according to Excelsior. Given the extensive network of pipelines running throughout the country and the government's inability to police them all, oil theft will likely remain a highly lucrative business for gangs, offering a relatively low-cost, low-risk source of revenue.