I came across this story via Dudge,

Mexican detained with anti-aircraft machine gun

A woman was arrested guarding an arsenal that included an anti-aircraft machine gun—the first weapon of its kind seized in Mexico, police said Tuesday.

The arsenal belonged to a group linked to the powerful Beltran Leyva drug cartel, federal police coordinator Gen. Rodolfo Cruz said. It also included ammunition, five rifles, a grenade and part of a grenade launcher.
…
Cruz said the confiscated .50-caliber, anti-aircraft machine gun can fire 800 rounds per minute and is capable of penetrating armor from more than 5,000 feet (1,500 meters). Police on a routine patrol Monday found the gun fitted atop an SUV at a house in northern Sonora state.

Take a good look at what they’re talking about:

Mexican authorities are not releasing information regarding the origin of this weapon.

Do any of my readers have any suggestions as to where this type of weapons come from?

UPDATE
Richard Fernandez replies,

That’s an M1919 .30 caliber machine gun, standard light machine gun used in World War 2. Browning was the designer, “The M1919 was manufactured during World War II by many different companies in the U.S. including General Motors and Rock Island Arsenal. In the UK production was chiefly by BSA” according to Wikipedia. It’s still in service in many countries, including many in Latin America.

Whoever can answer the question, Who did they buy it from? will have an answer as to where do the armaments the drug cartels are being purchased; It won’t be from some US weapons store.

UPDATE
The Houston Chronicle makes it sound like this was purchased at a US store:

On the very day officials confirmed President Barack Obama will appoint a border czar to combat drug cartels, Mexican police announced they confiscated a U.S.-made machine gun that could penetrate steel from a mile away, likely the most destructive weapon of its type ever seized in Mexico.

Prairie Pundit:

What should be clear is that this weapon was not purchased in a US gun store. I think the DEA agent is probably right in suggesting that it came from the Mexican army. If not there, it came from a black market sources in South and Central America. But the article tends to play into the myth that most of the insurgents weapons were bought over the counter in the US. This seems to be something the Obama administration is pushing in order to get greater control over gun sales. They are still pushing the discredited 90 percent myth on weapons in the hands of the insurgents coming from US dealers.

This gun is a good example of why that myth is invalid. It will not be sent to the US for testing because the Mexicans know it was not bought at a US gun store. They can probably match its serial numbers with a Mexican army unit.

Washington Times editorial:

The reality is that most guns that turn up in Mexico are not from the United States, do not have U.S. markings and therefore are not sent across the border to U.S. officials. The 90 percent figure reflects the number Mexico sends to the United State for tracing that turn out to be from the United States – not a percentage of the total number of all weapons seized in Mexico. Matt Allen, special agent of U.S. Immigration and Customs Enforcement (ICE), told Fox News that “the U.S. effort to trace weapons really only extends to weapons that have been in the U.S. market.” The vast number of seized weapons from other places is not counted.

Numerous sources of weapons supply Mexican crime syndicates. Mexico is a virtual arms bazaar filled with AK-47s from China, shoulder-fired rockets from Soviet-bloc manufacturers and fragmentation grenades from South Korea. In the past six years, more than 150,000 Mexican soldiers have deserted from the army, with many taking their standard-issue Belgian-made M-16s with them. Drug runners, who by trade are adept at smuggling, have the ability to sneak weapons across borders from anywhere they want.

And the funds to buy them with.