BROWNSVILLE, Texas - Federal agents are barely able to slow the river of American guns flowing into Mexico.

In two years, a new effort to increase inspections of travelers crossing the border has netted just 386 guns - an almost infinitesimal amount given that an estimated 2,000 slip across each day.

The problem came into sharp focus again last month when a U.S. Immigration and Customs Enforcement agent was killed on a northern Mexican highway with a gun that was purchased in a town outside Fort Worth, Texas.

Stopping the flow of American guns, bullets and cash has long bedeviled authorities on both sides of the border.

At a White House news conference in March 2009, Homeland Security Secretary Janet Napolitano joined President Obama in announcing plans to better help Mexico cope with a brutal drug war that has now killed more than 34,000 people since 2006.

"You've got to interdict the arms. You've got to stop them from going into Mexico," Napolitano said at the time.

Since then, Customs and Border Protection officers - who usually spend their days checking people and cars coming into the U.S. - have teamed up with Border Patrol agents and, sometimes, sheriff's deputies in border communities to scrutinize travelers leaving American soil.

They have made little progress.

In fiscal year 2009, Customs and Border Protection agents at all border crossings separating the 2,000-mile border, from Brownsville on Texas' Gulf Coast to San Diego, seized 107 guns.

The next fiscal year, ending Sept. 30, they seized 279. Those are the most-recent, border-wide figures available.

The Bureau of Alcohol, Tobacco, Firearms and Explosives reported seizing 2,633 guns in 2009 at its offices in the four southwestern border states, the most recent figures available, but those were captured before making it into border traffic - and even if they had, they would have amounted to a little more than a day's worth that get through.

A November 2008 study by The Brookings Institution, a Washington-based think tank, stated that 2,000 American guns are smuggled into Mexico each day. Compiled by a commission including ex-Mexican President Ernesto Zedillo and Thomas Pickering, a former ambassador to Russia and a senior State Department official during the administration of President Bill Clinton, the report was the last comprehensive estimate on the subject, though it did not include information on how that figure was reached.

Efforts to halt ammunition have been more successful. Outbound border inspections seized 93,141 rounds in the last fiscal year, more than four times the amount seized during the previous year.

Seizures of cash headed into Mexico fell from more than $37 million in fiscal year 2009 to about $27.4 million last year.

The ATF is now facing criticism after both CBS News and the nonprofit Center for Public Integrity reported that federal agents investigating gun-running by drug cartels allowed hundreds of guns purchased in the U.S. to go into Mexico.

The agency and prosecutors let the guns cross the border as they were building cases against traffickers, the center reported.

The ATF's work on the border highlights the tension between short-term operations aimed at arresting low-level straw buyers - legal U.S. residents with clean records who buy weapons - and long-term ones designed to identify who is directing the gun buys.

From September 2009 to July 31 of last year alone, the Mexican government seized more than 32,000 illegal weapons, even though purchasing guns in Mexico requires permission from the country's defense department, and even then buyers are limited to pistols of .38-caliber or less.

Not all those guns came from the United States - Mexican authorities have investigated reports that some were supplied by arms dealers in Israel and Belgium.

Many guns used to kill in Mexico never have their origins traced. Still, ATF has long estimated that of the weapons discovered at Mexican crime scenes which authorities do choose to trace, nearly 90 percent are eventually found to have been purchased in the U.S.

That was the case with the handgun found at the scene of the Feb. 15 drug cartel ambush that killed ICE Special Agent Jaime Zapata on a highway in Mexico's San Luis Potosi state. His partner was shot twice in the leg.

Authorities say the 7.62 mm pistol was purchased in Joshua, Texas. Three Dallas-area men - one accused of buying the gun, his brother and their neighbor - are facing federal weapons charges, although none related to Zapata's death. Investigators believe the trio on another occasion tried to sell dozens of weapons to the violent Zetas drug cartel, which recently saw one of its alleged bosses arrested in connection with Zapata's death.

"When a U.S. agent is killed by an illegally obtained U.S. gun, it really underscores the irony of our current policy," said David Shirk, director of the University of San Diego's Trans-Border Institute.

"We're not trying hard enough to stop the bad guys from using our weapons against us," he said.

Just how much money and manpower has been spent to detect so few southbound guns, bullets and bundles of U.S. dollars is difficult to pinpoint since the Department of Homeland Security doesn't comment on the number, location and frequency of its efforts.

However, Customs and Border Protection Commissioner Alan Bersin promised last April that at least $72.6 million would be allocated in fiscal year 2010 to hire 115 additional Customs officers and 144 new Border Patrol agents to bolster outbound operations.

The money would also have gone to improving southbound license plate-reading equipment and inspection technology.

Agency spokeswoman Kelly Ivahnenko said there were no figures on whether those staffing additions had been made, however, due in part to normal employment turnover. She said seizure numbers give only part of the picture and that stepped-up inspections have forced "smugglers to change their tactics, sometimes exposing them to other law enforcement agencies."

Complicating agents' hunt at the border is that most weapons and bullets are smuggled using "hormiga" or "ant" techniques, in which items are carried across in smaller amounts. Larger caches, stashed in 18-wheelers, are easier to find with X-ray equipment or sniffer dogs.