Here is a translated excerpt from this article in Milenio:
"The Public Security Secretary Hidalgo, Damián Canales Mena, confirmed the car bomb explosion which killed the commander of investigations assigned to Tula, Victor Peña, was a reaction of the criminal group known as "Los Zetas" in response to the deaths of two gunmen that took place weeks ago. Commander Victor Peña went to a spot on Interstate Tula-El Carmen to verify a citizen complaint about an abandoned car, minutes later exploded injuring three other officers... Similarly, the responsibility of the state Public Safety detective revealed that Peña had received death threats because of their investigations to disrupt criminal cells operating in the region of Tula." Link to Full Article
Analysis: This isn't the first instance we've seen in which DTOs have used car bombs to inflict damage against police. Two were set off in August last year in the Tamaulipas capital city of Ciudad Victoria. One was in front of a police station, and the other was in front of the Televisa new station. The first car bomb to be used since Calderón took office in 2006 exploded in Ciudad Juárez in July of last year. That one also caused little damage to anything other than the car itself.
Many people are concerned that these car bombs represent a scary escalation of drug war violence. I'll refer back to something Scott Stewart of Stratfor said in his analysis shortly after the Juárez explosion: "There's a difference between a car bomb and a bomb in a car." So far, the vehicles that have been used have contained relatively rudimentary explosive devices. They've also all been parked cars, and not even driven into a building or public area by remote control. It seems like a very elaborate way to target just one or a few people - namely police officers, government officials, or journalists - and not always very effective.
So with my analysis, I'm going to go the route of saying I believe the use of exploding cars is just a new way DTOs are trying to intimidate government forces. While they're not using true car bombs in the same way groups like the FARC or al-Qa'ida would, they still have a psychological impact because the image of a car exploding brings terrorist attacks to mind. Tossing grenades on the doorstep of a police or TV station has a similar effect; it may take out one or two people, or just cause damage to the building. However, stuff blowing up tends to scare the pants off people, and that's the real intended effect, in my opinion.
No, I don't like the fact that DTOs are exploring the option of using exploding cars as a psychological or physical weapon. However, I believe they're still a ways off from crossing that ever-more-gray line of going from heinous criminal activity to true terrorism. We'd have to see a car bomb with serious explosive impact going off in a crowded area with no other target other than innocent bystanders before we can start going down that road.