HARLINGEN, Texas (Reuters) - U.S. Border Patrol agent Reynaldo Zuniga was arrested last month lugging a bag of cocaine up from the Rio Grande, one of a growing number of law enforcement officers accused of taking bribes from drug gangs.

A U.S. Border Patrol agent holds a sniffer dog after inspecting a freight train near the Border Patrol traffic checkpoint in Kingsville, Texas July 1, 2008. REUTERS/Tomas Bravo

Former colleagues say Zuniga used to wait until agents in the south Texas town of Harlingen were distracted with paperwork, then slip down to the river and help smuggle in drugs from Mexico.

The increasing use of bribes by Mexican drug cartels to corrupt U.S. agents comes as Washington is sending $400 million to help Mexico’s army-led war on the trafficking gangs, whose brutal murders have surged to unprecedented levels.

“Zuniga was a good agent and a hard worker. I can’t understand why he would do this. We’re supposed to be protecting our borders,” said Border Patrol agent Daniel Doty, a former colleague.

Data on agents convicted of graft are not made public, but the U.S. government is probing hundreds of border corruption cases where a decade ago it saw a few dozen a year. The FBI-led Border Corruption Task Force says it is busier than ever.

“We’ve seen a sharp increase in investigations along the border over the past three years,” said Andy Black, who oversees the San Diego task force, near the busy border crossing of San Ysidro.

“We are talking about a minority of agents but they are a very significant threat, a weak link in efforts to secure the border.”

Some put the rise in bribery down to a recent tightening of border controls and a jump in hiring new agents. Smugglers can offer hundreds of thousands of dollars to get past the heavily policed border with drugs and immigrants -- much more than a border agent or sheriff makes in a year.

Gangs also often use attractive women as bait, setting a “honey trap” to entice officials.

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“I was offered sex to let a woman across the Rio Grande, but I have a family, I turned her down,” one agent told Reuters as his sniffer dog searched a freight train for immigrants and drugs in the Texan borderlands, steamy with tropical rain.

“BAD AGENTS”

Corruption south of the border is a major hurdle to Mexican President Felipe Calderon’s quest to crush drug gangs, with up to half the country’s police thought to be crooked. Spiraling drug violence has killed 1,700 people in Mexico this year.

U.S. anti-drug officials have pointed to higher street cocaine prices as proof of tighter border controls.

But the campaign is weakened by cases like that of a border agent and his brother in Texas who netted $1.5 million by letting tonnes of marijuana through checkpoint inspection lanes from 2003 to 2005.

Trafficking drugs and people generates billions of dollars a year. Powerful gangs use crooked officials well beyond the border to open smuggling lanes into the United States.

In one case showing the breadth of the problem, two California-based employees of Wackenhut, a contractor that transports detained illegal immigrants, were charged last month with freeing them for $2,500 each.

Also in June, police arrested a Los Angeles attorney for U.S. Immigration and Customs Enforcement (ICE) for allegedly accepting huge bribes to issue green cards and other papers.

“This was an amazing compromise of our system and its integrity,” said Paul Layman, a special agent who oversees ICE’s corruption investigations in the western United States.

“Smugglers are willing to do anything to get people into the country, they will move anything for a dollar.”

U.S. Customs inspector Richard Elizalda, arrested in 2006, was paid $70,000 to let through hundreds of immigrants after a persuasive female smuggler he met at the San Ysidro crossing became his lover.

A sudden influx of Border Patrol agents may have worsened the problem. The number of agents along the border has jumped to more than 14,700 now from less than 9,000 four years ago.

Agents receive intense training and ethics courses, but some officials worry about the screening process.

“Just given the increases, the odds are you’ll get more bad agents,” said Paul Charlton, a former U.S. Attorney for Arizona.