SAN DIEGO -- At first, Luis F. Alarid seemed well on his way to becoming a customs agency success story. He had risen from a childhood of poverty and foster homes, some of them abusive, earned praise and commendations while serving in the Army and the Marines, including two tours in Iraq, and returned to Southern California to fulfill a goal of serving in law enforcement.
But, early last year, after just a few months as a customs inspector, he was waving in trucks from Mexico carrying loads of marijuana and illegal immigrants. He pocketed some $200,000 in cash that paid for, as far as the government could tell, a $15,000 motorcycle, flat-screen televisions, a laptop computer and more.
Some investigators believe that Mr. Alarid, 32, who was paid off by a Mexican smuggling crew that included several members of his family, intended to work for smugglers all along. At one point, Mr. Alarid, who was sentenced to seven years in federal prison in February, told investigators that he had researched just how much prison time he might get for his crimes and believed, as investigators later reported, that he could do it "standing on his head."
Mr. Alarid's case is not the only one that has law enforcement officials worried that Mexican traffickers -- facing beefed-up security on the border that now includes miles of new fencing, floodlights, drones, motion sensors and cameras -- have stepped up their efforts to corrupt the border police.
They research potential targets, anticorruption investigators said, exploiting the cross-border clans and relationships that define the region, offering money, sex, whatever it takes. But, with the border police in the midst of a hiring boom, law enforcement officers believe that traffickers are pulling out the stops, even soliciting some of their own operatives to apply for jobs.
"In some ways," said Keith Slotter, the agent in charge of the F.B.I.'s San Diego office, "it's like the old spy game between the old Soviet Union and the U.S. -- trying to compromise each other's spies."
James Tomsheck, the assistant commissioner for internal affairs at Customs and Border Protection, and other investigators said they had seen many signs that the drug organizations were making a concerted effort to infiltrate the ranks.
"We are very concerned," Mr. Tomsheck said. "There have been verifiable instances where people were directed to C.B.P. to apply for positions only for the purpose of enhancing the goals of criminal organizations. They had been selected because they had no criminal record; a background investigation would not develop derogatory information."
During a federal trial of a recently hired Border Patrol agent this year, one drug trafficker with ties to organized crime in Mexico described how he had enticed the agent, a close friend from high school in Del Rio, Tex., who was entering the training academy, to join his crew smuggling tons of marijuana into Texas.
The agent, Raquel Esquivel, 25, was sentenced to 15 years in prison last week for tipping smugglers on where border guards were and suggesting how they could avoid getting caught.
The smuggler, Diego Esquivel, who is not related to the agent, said he told her that her decision to enter the academy was a good career move and, he said, "I thought it was good for me, too."
Under the Bush administration, the United States has spent billions of dollars -- $11 billion this year alone for Customs and Border Protection -- to tighten the border between the United States and Mexico, building up physical barriers and going on a hiring spree to develop the nation's largest law enforcement agency to patrol the area.
But the battle for survival among cartels in Mexico, in which thousands of people, mostly in the drug trade or fighting it, have been killed, has only led drug traffickers to redouble their efforts to get their drugs to market in the United States.
Along the border, many residents have family members on both sides. Generations of residents have been accustomed to passing back and forth relatively freely, often daily, and exchanging goods, legal or not.
Federal officials believe that drug traffickers are seeking to exploit those ties more than ever, urging family and friends on the American side to take advantage of the hiring rush for customs agents. The majority of agents and officers stay out of crime. But smuggling can be appealing. The average officer makes $70,000 a year, a sum that can be dwarfed by what smugglers pay to get just a few trucks full of drugs into the United States.
Right now, only a fraction -- 10 percent or so -- of Customs and Border Protection recruits are given a polygraph screening that federal investigators say has proved effective in weeding out people with drug ties and other troublesome backgrounds. Officials say they do not have the money to test more recruits.
In years past, new hires rarely served in the areas where they had grown up, but recently that practice has been relaxed somewhat to attract more recruits, said Thomas Frost, an assistant inspector general at the Department of Homeland Security. Mr. Frost and other internal affairs veterans say that has made it easier for traffickers.
Mr. Tomsheck said that several prospective hires had been turned away after investigators suspected that they had been directed to Customs and Border Enforcement by drug trafficking organizations, and that several recent hires were under investigation as well, though he declined to provide details.
As one exasperated investigator at the border put it, "There is so much hiring; if you have a warm body and pulse, you have a job."
The F.B.I. is planning to add three multiagency corruption squads to the 10 already on the Southwest border, and the Department of Homeland Security's inspector general, the department's primary investigative arm, has also added agents. But such hiring has not kept up with the growth of the agency they are entrusted to keep watch over.
Over all, arrests of Customs and Border Protection agents and officers have increased 40 percent in the last few years, outpacing the 24 percent growth in the agency itself, according to the Department of Homeland Security inspector general's office. The office has 400 open investigations, each often spanning a few years or more.
Keith A. Byers, who supervises the F.B.I.'s border corruption units, said corruption posed a national security threat because guards seldom verify what is in the vehicles they have agreed to let pass, raising concerns "they could be letting something much more dangerous into the U.S."
Most corrupt officers gravitate to smuggling illegal immigrants, rationalizing that is less onerous than getting involved with drugs, investigators say.
But Mr. Byers and others point to a string of drug-related cases that make them wonder if the conventional wisdom is holding.
Margarita Crispin, a former customs inspector in El Paso, pleaded guilty in April 2008 and received a 20-year prison sentence in what the F.B.I. considers one of the more egregious corruption cases.
Through a succession of boyfriends and other associates with ties to major drug trafficking organizations in Ciudad Juarez, Mexico, Ms. Crispin helped smuggle thousands of pounds of marijuana over three years, almost from the time she began working for the agency.
She waved off drug-sniffing dogs in her lane, complaining she was afraid of them, although investigators later learned she had had dogs as pets.
"She is someone who from the beginning said this would be a good job to help the people I am associated with," Mr. Byers said.
Just last month, Martha Garnica, a 12-year Customs and Border Protection employee near El Paso, was charged with bribery and marijuana smuggling in concert with traffickers in Ciudad Juarez.
Ms. Garnica's 21-year-old daughter had also sought a job with the Border Patrol, in what investigators deemed a suspicious move given her mother's alleged involvement in the drug trade. The daughter, testifying in court last week, admitted she had lied on the application both about being a United States citizen and about owning property in Mexico. A spokesman for the United States Attorney's Office in El Paso declined to comment.
Mr. Alarid's history in the military probably made him seem like a good candidate for the customs job. But he had a tangled family history. According to court papers, both his parents were drug addicts.
Mr. Alarid was born in Tijuana, Mexico, but raised largely in foster homes in Southern California. He emerged from high school a track star and, over the next 10 years, did stints in the Marines and the Army, drawing praise from commanders for his dedication and service.
"I would willingly trust Luis with my life," Sgt. Maj. Michael W. Abbey of the Army wrote in a letter to the judge before Mr. Alarid was sentenced in February.
Mr. Alarid began working at the border in San Diego in October 2007. In his guilty plea, he admitted that he had started smuggling in February 2008. He was arrested three months later.
Mr. Alarid would wave in vehicles that should have raised suspicion, either because their license plates were partly covered or because the plates did not belong to the vehicle, something he would have seen on the computer screen in his inspection booth.
Before reporting to his lane, he would go out to the employee parking lot to use his cellphone, which federal agents believe was his way of telling the smugglers which lane to approach.
At his sentencing, all involved -- the prosecutors, the judge, his lawyer -- expressed bewilderment at the turn in Mr. Alarid's life. But in an interview, a family member who was not part of the case said Mr. Alarid had mounting gambling debts and, despite it all, had always sought a bond with his biological mother.
Still, Judge Janis L. Sammartino accepted the government's argument that a deterrent message needed to be sent.
"I do think that the public, for a while at least, needs to be assured that who we have at the border are 100 percent individuals of integrity," she said. "I think you were at one time. I don't know what went wrong for you, sir, and I hope that you attain that again."
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