No other state has produced more guns seized by police in the brutal Mexican drug wars than Texas. In the Lone Star State, no other city has more guns linked to Mexican crime scenes than Houston. And in the Texas oil town, no single independent dealer stands out more for selling guns traced from south of the border than Bill Carter.

Carter, 76, has operated four Carter's Country stores in the Houston metropolitan area over the past half-century. In the past two years, more than 115 guns from his stores have been seized by the police and military in Mexico.

As an unprecedented number of American guns flows to the murderous drug cartels across the border, the identities of U.S. dealers that sell guns seized at Mexican crime scenes remain confidential under a law passed by Congress in 2003.

A year-long investigation by The Washington Post has cracked that secrecy and uncovered the names of the top 12 U.S. dealers of guns traced to Mexico in the past two years.

Eight of the top 12 dealers are in Texas, three are in Arizona, and one is in California. In Texas, two of the four Houston area Carter's Country stores are on the list, along with four gun retailers in the Rio Grande Valley at the southern tip of the state. There are 3,800 gun retailers in Texas, 300 in Houston alone.

"One of the reasons that Houston is the number one source, you can go to a different gun store for a month and never hit the same gun store," said J. Dewey Webb, special agent in charge of the Houston field division of the Bureau of Alcohol, Tobacco, Firearms and Explosives. "You can buy [a 9mm handgun] down along the border, but if you come to Houston, you can probably buy it cheaper because there's more dealers, there's more competition."

Drug cartels have aggressively turned to the United States because Mexico severely restricts gun ownership. Following gunrunning paths that have been in place for 50 years, firearms cross the border and end up in the hands of criminals as well as ordinary citizens seeking protection.

"This is not a new phenomenon," Webb said.

What is different now, authorities say, is the number of high-powered rifles heading south - AR-15s, AK-47s, armor-piercing .50-caliber weapons - and the savagery of the violence.

Federal authorities say more than 60,000 U.S. guns of all types have been recovered in Mexico in the past four years, helping fuel the violence that has contributed to 30,000 deaths. Mexican President Felipe Calderon came to Washington in May and urged Congress and President Obama to stop the flow of guns south.

U.S. law enforcement has ramped up its focus on gun trafficking along the southwestern border. Arrests of individual gunrunners have surged. But investigators rarely bring regulatory actions or criminal cases against U.S. gun dealers, in part because of laws backed by the gun lobby that make it difficult to prove cases.

All of the stores among the top 12 have had double-digit traces of "crime guns" to their stores from Mexico, a statistic that can be a red flag for investigators. A high number of traces does not necessarily signal wrongdoing. It could be the result of sales volume, geography or clientele. Carter's Country, for instance, is the largest independent gun retailer in the region. Most experts and ATF officials agree that the majority of dealers are law-abiding.