MEXICO CITY – Deep inside a heavily guarded military warehouse, the evidence of Mexico's war on drug cartels is stacked two stories high: tens of thousands of seized weapons, from handguns and rifles to AK-47s, some with gun sights carved into the shape of a rooster or a horse's head.

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The vault nestled in a Mexican military base is the government's largest stash of weapons — some 88,537 of them — seized from brutal drug gangs. The Associated Press was recently given rare and exclusive access to the secure facility.

The sheer size of the cache attests to the seemingly hopeless task of ever sorting and tracing the guns, possibly to trafficking rings that deliver weapons to Mexico. And security designed to keep the guns from getting back on the streets is so tight that even investigators have trouble getting the access they need.

The warehouse — on a main drag in northeastern Mexico City near the horse racing track — is surrounded by five rings of security. There are two military guards at the door and five more are in the lobby. Inside, another 10 soldiers sort, clean and catalog weapons. Some are dismantled and destroyed, a few assigned to the Mexican military.

The guns are stacked to the two-story ceiling in a warehouse the size of a small Wal-Mart. The rifles lie on 22 metal racks; the pistols hang from metal poles by their triggers.

The cavernous warehouse is impeccably clean, the only smell coming from the coffee the soldiers prepared for their rare visitors. The clash of metal and sounds of the soldiers at work echo off the walls.

The security, bolstered by closed-circuit cameras and motion detectors, makes the warehouse practically impenetrable, said Gen. Antonio Erasto Monsivais, who oversees the armory.

In all, the military has 305,424 confiscated weapons locked in vaults, just a fraction of those used by criminals in Mexico, where an offensive by drug cartels against the military has killed more than 10,750 people since December 2006. But each weapon is a clue to how the cartels are getting arms, and possibly to the traffickers that brought them here.

The U.S. has acknowledged that many of the rifles, handguns and ammunition used by the cartels come from its side of the border. Mexican gun laws are strict, especially compared to those in most U.S. border states.

The Mexican government has handed over information to U.S. authorities to trace 12,073 weapons seized in 2008 crimes — particularly on guns from large seizures or notorious crimes.

But the Bureau of Alcohol, Tobacco, Firearms and Explosives, which handles the U.S. investigations, is at the mercy of local Mexican police for the amount and quality of the information.

"Many of these rural municipalities that may come into a gun seizure ... may not even know anything about tracing guns," ATF spokesman Thomas Mangan said.

A police officer in Mexico submits a description, serial number and distinctive markings of the gun. The weapons are then turned over to the military for storage in one of a dozen armories such as the one in Mexico City.

When U.S. investigators need additional details, as they often do, the request goes back to the original police officer, who must retrieve the gun from a military vault — sometimes hundreds of miles away.

Mexican police must ask permission each time they need to look at a stored gun, Monsivais said. Even if that permission is granted, the investigator cannot go past the metal fencing separating a reception desk and the shelves holding the guns. A soldier has to bring out the requested weapons.

The security, language differences and bureaucracy add up to a painstaking process, said J. Dewey Webb, special agent in charge of the ATF's Houston Field Division.

"The military does a very good job when the weapons come into their custody of securing them," he told the AP. "Because of the systems in Mexico, it's very difficult for us to get in."

Webb said recent talks between the two countries were beginning to ease access, but also noted other problems.

Many mistakes are made because of difficulty translating technical terms about firearms, Webb said. A Spanish-language version of eTrace, the Web-based method of submitting tracing information, won't be available until next year.

About a third of the guns submitted for tracing in 2007 were sold by licensed U.S. dealers.

U.S. agents need the information to track the gun back to the manufacturer and determine when it was made and what wholesaler it was shipped to, ATF spokeswoman Franceska Perot said. Agents follow the gun to the local licensed dealer who sold it and determine the buyer.

ATF offices around the U.S. are swamped with tracing requests, trying to determine who actually bought the weapons and whether they were part of a firearms trafficking scheme. The ATF has sent an extra 100 agents to Houston to help unclog the 700-weapon backlog as part of its Project Gunrunner.

The seized weapons are kept in the vaults as long as they are needed as evidence, Monsivais said. Most have been there for years, an indication of how slow criminal investigations proceed and how few crimes are ever solved.

Indeed, the ATF gave the AP data showing the average "time to crime" — the time between when a gun was sold and when it was seized in a crime — is 14 years.

That's an average of four years longer than guns in American crimes, the ATF said. The older the street age, the harder it can be to track how the gun wound up at a crime scene.

When the criminal investigations are complete, most of the weapons are destroyed and melted down. Some of the more powerful arms, such as M16 machine guns and sniper rifles, are added to the military's own arsenal. Showpieces are destined for museums.

Most of the guns traced were originally sold by U.S. dealers in border states, with more than half purchased in Texas. Not only does Texas have the most gun dealers of any state, it makes up 1,200 miles of the 2,100-mile U.S.-Mexico border, with many of the established drug and trafficking routes.

Details on the 2008 tracing requests are not yet available.

It's less clear how cartels are getting military-grade weapons. Amid the shelves of pistols and rifles, there is a 9 mm grenade launcher and a portable shoulder-fired anti-tank rocket launcher.

Such military-grade weaponry represents a tiny fraction of the seized weapons. But Monsivais said he's most worried about the rising caliber of assault rifles and semi-automatic guns that have been found.

"There are weapons that have a lot of firepower and great penetration, like the .50-caliber Barrett ... which can penetrate armored vehicles, body armor, and that normally only militaries use," Monsivais said.

Thirty percent of AK-47 assault rifles seized have been modified to become fully automatic. He said about three of every 1,000 AR-15 assault rifles have been modified to take .50-caliber bullets, the kind of high-powered ammunition designed for sniper rifles.

"In my experience, I had never seen a modified AR-15 rifle," Monsivais said. "It's something new, and it is to a certain extent worrisome that they can have and use this type of weapon."

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Roberts reported from San Antonio. Associated Press writers Alexandra Olson in Mexico City and Juan A. Lozano in Houston contributed to this story.

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