JUAREZ, Mexico (CNN)  -- A deadly trade is occurring along the U.S. border with Mexico, federal officials say -- a flood of guns, heading south, used by drug thugs to kill Mexican cops.

Authorities recently seized these .50-caliber bullets, already belted to be fed into a machine gun.

In Mexico, guns are difficult to purchase legally. So, officials say, weapons easily purchased in the United States are turning up there.

"The same routes that are being used to traffic drugs north -- and the same organizations that have control over those routes -- are the same organizations that bring the money and the cash proceeds south as well as the guns and the ammunition," says Bill Newell, a special agent with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives.

Police in Mexican border towns fear for their lives, and with good reason. Five high-ranking Mexican police officials have been killed this year in what Mexican officials say is an escalating war between police and drug cartels.

In Juarez, Mexico, just across the border from El Paso, Texas, a police commander was gunned down in front of his home. The weapon used to kill Cmdr. Francisco Ledesma Salazar is believed to have been a .50-caliber rifle. The guns are illegal to purchase in Mexico but can be obtained just north of the border at gun shows and gun shops in the United States.

ATF special agent Tom Mangan says the .50-caliber rifle has become one of the "guns of choice" for the drug cartels. The weapon fires palm-sized .50-caliber rounds that can cut through just about anything.

Mangan showed CNN the power of the rifle on a gun range near Phoenix, Arizona. The weapon, a Barrett, was seized in an ATF raid. A round fired from 100 yards away tore through a car door and both sides of a bulletproof vest like those used by Mexican police.

"There's nothing that's going to stop this round," Mangan says.

The rifle was intercepted as it was being smuggled into Mexico. Mangan says investigators believe four others already had passed through the border.  Watch how the weapons fuel a little-known war Â»

The ATF has been trying to help Mexican police by cracking down on illegal purchases of guns and ammunition. Operation Gunrunner has led to several arrests and seizures of guns and ammo. But the operation has mainly shown just how big a problem exists, authorities say.

One recent seizure in a Yuma, Arizona, storage locker yielded 42 weapons and hundreds of rounds of .50-caliber bullets already belted to be fed into a machine gun-style weapon.

The guns confiscated included AK-47 rifles and dozens of Fabrique National pistols. The semiautomatic pistols fire a 5.7-by-28 millimeter round, which is technically a rifle round, according to the ATF. Newell says the round has a special nickname in Mexico. "It's called 'mata policias,' or 'cop killer,' " he says.

Mexican authorities along the border recently met with their counterparts in the United States, hoping more cooperation will lead to more arrests of criminals and fewer killings of Mexican police officers.

Guillermo Fonseca, Mexico's regional legal attachÃ© for the West Coast, told CNN the violence in his country is "problem number 1" -- and police in his country are outgunned. Officers in Mexico lack heavy firepower, he says. With the presence of large-caliber weapons from the United States, drug cartels and criminals have the advantage in what he says is basically a war. Part of the solution, he says, is for the United States to give Mexico more information about who is selling these guns illegally in the United States. Then Mexico could go after the buyers.

"We have access to systems to trace guns that have been smuggled into Mexico, and that has worked very well," Fonseca told CNN. "We need more information about the people who are actually purchasing the guns. We need to prosecute those people, to convict those people. In our opinion, that's the next step we have to take."

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