The trial here of two former U.S. Border Patrol agents accused of human trafficking is focusing renewed attention to corruption in the agency.

Federal prosecutors have charged brothers Raul and Fidel Villarreal with conspiracy, human smuggling, bribery and witness tampering, among other offenses.

The case is not without irony, since Raul Villarreal was one of the spokesmen for the Border Patrol in San Diego and frequently warned about the dangers of immigrant trafficking.

The indictment says Raul became the leader of a band of traffickers that operated up until 2008 and collected payments of up to $12,000 per immigrant.

"They decided to make the border work for them. Instead of defending the border, they decided to profit from it," Assistant U.S. Attorney Timothy Salel said in his opening statement at the trial.

However, Raul's defense attorney, J. David Nick, said that there is little evidence linking his client to migrant trafficking and that the case rests on the doubtful testimony of paid witnesses and members of an organization of traffickers who agreed to cooperate with the authorities.

Corruption cases involving Border Patrol agents have increased in recent years, reaching 60 in 2011, while the number of investigations surged from 244 in 2006 to 870 last year, according to the Department of Homeland Security.

Expets say this is due both to the rapid growth of the Border Patrol and to the inducements or threats from criminal organizations.

Most of the 132 employees who have been charged or convicted were assigned to the southwest border.

About half of the roughly 24,000 agents who worked along the southern border were hired during the past five years, and the Border Patrol is scheduled to hire 2,500 additional officers this year.

Victor Clark Alfaro, a professor at San Diego State University who also heads the Tijuana-based Binational Human Rights Center, told Efe that the corruption cases arise due to inadequate supervision resulting from hiring so many new agents.

He said that the relative security offered in crossing the frontier when corrupt agents are participating in the operation translates into an increase in the price paid by each undocumented migrant.

Both Raul and Fidel Villarreal resigned their positions, allegedly after they were alerted about the existence of a federal investigation, and fled to Mexico, where they were captured in 2008 and extradited to the United States in 2009.

Raul Villarreal had a high profile media presence on both sides of the border.

He began working for the Border Patrol in 1995 and, authorities say, he created the criminal organization in April 2005 before later that year recruiting his brother Fidel, who had joined the Border Patrol in 1998.

Prosecutors say the organization grew to include 14 members, among whom were former Tijuana police chief Gerardo Santiago Prado, who was murdered in 2006. EFE