What are the fake police checkpoints in Mexico looking for?

Question

I'm not entirely sure if this is the right place to ask this, but my reason for asking is travel; and travel safety (as well as sheer curiosity).

Last week I came across a clearly¹ fake² police checkpoint in Mexico, and they had me (and my two passengers) get out of the car, did a superficial search of the car, then sent us on our way.

This makes me wonder:

What are these unofficial checkpoints looking for? And the followup question How can I reduce the chances of being stopped/detained/interrogated/robbed/kidnapped/murdered/raped/whatever at one?

I've heard many unsubstantiated rumors:

• They're "only" looking for rival criminals (probably drug cartels)
• They're looking for valuables to steal
• They're looking for wealthy people to kidnap

I also realize there may not be a single correct answer to this question, as each checkpoint may be looking for something different; but hopefully a generally useful answer can be provided.

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¹I was an hour from the nearest town, but the uniforms all said Policía Municipal, and their vehicles were faded (police vehicles are usually very nice in Mexico), and from the state of Guanajuato, although we were in San Luis Potosí. This would be like driving through Texas and coming across a police checkpoint manned by the LAPD. Pretty fishy.

²This question is about criminals (I believe usually drug cartels) impersonating police officers.

Answer 1

The category on Driving in Mexico on Wikitravel has a couple of things to say:

Never drive above the speed limit or run stop signs/red lights as Mexican police will use any excuse to pull over tourists and give you a ticket. If pulled over by a police officer soliciting a bribe, do not pay the amount requested, but pull out USD$50 or 500 pesos, and explain that it is all you have. This technique has worked in the past (but it does not work in Mexico City), but it is corruption. Corruption also is a crime in Mexico, so make a conscious choice. The fine for speeding could be as much as US$100, depending on the city.

As of April 2011, Police across the country are cracking down on drunken driving, particularly in Mexico City, the larger cities and the beach resorts. There are random checkpoints throughout the country in which every driver has to stop and take an automated inebriation test. If you fail, you will end up in a Mexican prison. If you wouldn't drive drunk back home, don't do it in Mexico.

So in this case, it would imply they're looking for three things:

• drunk drivers
• speeding drivers
• drugs
• the opportunity to elicit a bribe (also can be coupled with the previous three items)

That is for REAL Mexican police stops.

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Now as for fake police stops, the US Consulate has this to say, as of 2010:

U.S. officials have clarified an earlier travel advisory that warned Americans against traveling after dark to Puerto Penasco, Mexico, also known as Rocky Point.

The U.S. Consulate in Nogales, Mexico, had issued the warning because travelers have been stopped by phony Mexican police roadblocks.

In an e-mail obtained by the Arizona Daily Star, the consulate stands behind its advisory, but clarifies that the warning only advises against traveling on the highway to Puerto Penasco at night and didn't apply to other highways in the Mexican state of Sonora or in the city of Rocky Point itself.

A few other results seem to indicate some prejudice against people from the state of Arizona specifically, as seen on this thread:

...there's people down in Mexico looking for people from Arizona. Fake police and fake roadblocks. I doubt anything good will happen to someone who is found out to be from Arizona.

There was a tourism ad released not too long ago that Sheriff Arpaio sees as threatening to AZ citizens, and this may be related to that.