We are also in the dark about when they were killed. One bus driver told El Universal that attacks had been happening in the area for weeks before the recent cases that caused the uproar and started the search for the bodies.

Aside from the sixteen police officers arrested, there is little information about the others who have been captured, who, as of April 14, numbered eighteen. The government is keeping a tight lid on its investigation, and the new attorney general read from a pre-prepared release rather than take questions on the case. The only information authorities have revealed is that those captured are members of the Zetas criminal organization.

But the Zetas have become as much a brand name as an organized criminal gang. Depending on who you ask, the Zetas have from 1,000 to 25,000 soldiers, so it's hard to determine what the motives were and exactly what part of the “Zetas” was calling the shots in this particular case.

The theories about the killings, in no particular order, range from robbery to forced recruitment, kidnapping, the interception of potential enemy soldiers, and what the Mexicans like to call “heating up the plaza.”

Robbery seems improbable. There are easier and less risky ways to make money if you are part of a large or even a small criminal enterprise. There is also little reason to expose yourself by attacking buses, with multiple witnesses, in order to secure very small sums of money from the lower classes who use this mode of transportation.

Forced recruitment also seems unlikely. Although there have been reports of Guatemalan migrants being forced into service, these organizations seem to have little problem attracting new members locally. Recruiting blindly does not seem a sound way to ensure loyalty to the group, and their efforts did not bear much fruit, judging by the hundred-plus bodies in the ground.

Kidnapping is a possibility. Organizations like the Zetas have branched into kidnapping, targeting illegal migrants on their way north, and San Fernando has a history of such incidents. Last year 72 migrants were killed by suspected members of the same group.

In the highly convoluted official explanation, authorities said the gang was using kidnapping to recruit new members, which ended in tragedy when the migrants refused to work for them. (This also seems highly unlikely, but it is a story that authorities from two different governments told InSight is backed up by at least two surviving witnesses.)

So perhaps this is another abduction case. The Guatemalan and Salvadoran governments sure want to know, and are asking Mexico to provide information on the victims.

However, kidnapping would have involved contact with families of the victims to demand some sort of ransom. While some of those in the graves may have been kidnapped, there are also reports of Mexican families sending DNA to authorities to see if their disappeared relatives are among the dead. This suggests that they have not had news of their relatives since their disappearances, which does not fit with the kidnapping theory.

The Zetas may have also noticed that their enemies were bringing in reinforcements. Tamaulipas is the setting for one of the most epic fights taking place in Mexico right now, with the Gulf Cartel battling the Zetas, its former henchmen, almost to the square foot. The prize is access to the United States, but little else. Tamaulipas is a relatively poor state apart from its use as a drug route.

The Gulf has teamed with other cartels in this battle including the Familia Michoacana and the Sinaloa Cartel. It's possible that the Zetas sniffed out a plan by this alliance to move extra soldiers via bus into the Gulf stronghold, which is located further north in the cities of Reynosa and Matamoros.

Discovering the identities of the soldiers who were coming may have proven too difficult, so perhaps the killers simply searched for young males from certain areas where their rivals operate; or maybe just young males. But families from all over the country and several foreign countries are seeking to find out if loved ones are victims, so this explanation does not seem to fit.

Lastly, it could be that rival groups are trying to “heat up the plaza.” In general terms, this means creating a mess so that the authorities come swooping in and disrupt business. In this specific case, it seems like an excessive way to achieve that goal, even by the warped narco-world standards.

Perhaps the deeper question that needs asking is why this happened. Is it part of a larger strategy that the Zetas are implementing, a variation of “heating up the plaza” by drawing authorities into a region, so they can do business in another part of the country?

One analyst told InSight that this was his theory, arguing that the Zetas’ commander Heriberto Lazcano Lazcano controls these players like General George Patton once did the Allied troops in Europe in World War II.

Or is this part of a larger trend of the fragmentation of these groups? It could be a step in a process in which increasingly independent or semi-independent wings of larger organizations like the Zetas take their own decisions, however poorly thought-out or devastating they may be. By this theory, over time, as they grow and become more independent, their interests become centered on their own survival, not that of their bosses.

InSight leans towards the latter explanation. In part this is because it speaks to the chaos in places like Tamaulipas -- a chaos that comes when an organization like the Zetas grows rapidly and loosely, and does not have control over all its factions. These sub-groups branch into other criminal activities, with or without their bosses’ approval.

The atomization of drug gangs has led to similarly devastating consequences in places like Juarez, where smaller groups battle for ever-smaller pieces of the criminal pie.

This theory may also help explain why there are close to 30 shallow graves with hundreds of victims. They could have been killed and buried there due to a combination of all the above factors, in a series of crimes perpetrated by a group or groups that have no unifying goal other than the scramble to survive in an increasingly crowded and competitive criminal world.

There may be more mass graves waiting to be discovered. Authorities need to start scouring the rest of the country with the same fine-tooth comb they are now using in Tamaulipas.