At first, some thought the explosions may have been caused by accidents, but investigators at the bombing sites in north-central Mexico found evidence of sophisticated explosives, a leftist insurgent group from southern Mexico, the Popular Revolutionary Army (EPR) claimed responsibility. President Felipe Calderon pledged to protect the infrastructure of the state-owned energy , Petroleos Mexicanos, called Pemex for short, and he dispatched military units to patrol various pipeline routes.

But the attacks remain a mystery, and speculation continues as who was really responsible. The EPR had never ventured that far north before and never appeared to have access to the explosives and bomb-making skills that employed in these attacks.

Political analyst Ana Maria Salazar says she remains baffled by the bombings.

"I do not know what to make of it," she . "That is part of the problem when you have these acts that are claimed to have been done by armed groups in Mexico, you always have question in the back of your mind - who did it?"

She says it is possible the EPR might have carried out the attacks in conjunction other leftist groups currently protesting in the southern state of Oaxaca over various issues. But she says she cannot rule out the possibility that a foreign group might have had a hand in it.

"There was a threat posted on a Web site a couple of months ago, supposedly by al-Qaida, that threatening all the countries providing petroleum to the United States, which included Mexico and Venezuela, which made Hugo Chavez very upset, by the way," she added. " there is that possibility."

Mexico City-based independent energy analyst David Shields is among those who see another possible culprit behind the bomb attacks, the powerful and drug trafficking cartels, against which President Calderon launched an offensive shortly after taking office last December.

"Once it was confirmed that these were terrorist attacks, first all we thought of the drug cartels as the likely authors of these attacks," he explained. "I think we cannot rule out altogether that they were ."

Shields says government efforts to protect the state-owned energy company's pipes and facilities are necessary, but he says posting soldiers along the pipeline routes will not complete protection.

"We know that Pemex has infrastructure of all kinds of pipelines, something like 60,000 kilometers of pipelines all over the country, as well as kinds of facilities," he added. "I think it would even be out of the question to post a soldier every kilometer. It would be a massive of human resources to have people all over the country wherever there are pipelines, wherever there is infrastructure."

Shields says the Mexican government has provided adequate to its oil production and export infrastructure, even employing missile-equipped naval vessels to patrol waters near offshore oil platforms. But he says pipelines serving Mexico's own are much more vulnerable.

"Certainly, none of us are ruling out the likelihood of more attacks, but also what we have to realize is that these on domestic pipelines are affecting the domestic market, they are not affecting exports," he noted. "It would have to be a very different kind of attack a more sophisticated attack on oil export facilities for oil exports to be interrupted. That seems to be an unlikely scenario."

While Mexico's oil export system be safe, a continued threat to its domestic energy supply network could cripple important industries and discourage foreign investors who are considering placing plants here. Security who have examined evidence from the bombings say the perpetrators knew the attacks would disrupt important industries and that they probably had information from someone inside as to which pipes to bomb for the most effect.