PHOENIX, June 2 — With a proposed major expansion, the Border Patrol may soon overtake the F.B.I. as the largest federal law enforcement agency. But the stepped-up mission comes as the Border Patrol wrestles with recruitment and training difficulties, and several agents face accusations of misconduct.

In response to concerns, the inspector general's office of the Homeland Security Department, which oversees the Border Patrol, said it would audit the agency's recruitment, hiring and training practices to determine if it can handle the rapid expansion. A spokeswoman, Tamara Faulkner, said the review could begin this month.

David V. Aguilar, the chief of the Border Patrol, told Congress last week that the extraordinary growth was vital to national security, particularly as the authorities seek to clamp down on illegal crossings along the Mexican border. The agency has swelled to more than 11,000 agents from 4,000 15 years ago, with 6,000 more proposed by President Bush by 2008 as a cornerstone of his immigration overhaul.

"The nexus between our post-Sept. 11 mission and our traditional role is clear," Chief Aguilar said. "Terrorists and violent criminals may exploit smuggling routes used by migrants to enter the United States illegally and do us harm."

But as the Border Patrol seeks more agents, its training academy in Artesia, N.M., needs expansion, and some watchdog groups question its ability to prepare so many new agents in so little time. As a temporary measure, thousands of National Guard troops will soon be dispatched here in Arizona and elsewhere along the 2,000-mile border to assist with logistics and support work.

"This is not something where you can snap your fingers and have thousands go on the job," said Deborah W. Meyers, an analyst at the nonpartisan Migration Policy Institute. "It is a demanding job, and training is important and intense."

Big buildups in border security in the 1990's coincided with a rash of embarrassing disclosures about wayward agents and questions about how well the agency screened recruits. Those concerns have surfaced again as several agents have been accused of misconduct and immigrant smuggling, including one agent from Mexico who was hired in 2002 even though he is not a United States citizen, as is required.

In January, the Mexican agent, Oscar Antonio Ortiz, who had falsely claimed citizenship on his job application, pleaded guilty to charges of immigrant smuggling and other crimes and is awaiting sentencing. Mr. Ortiz, 28, had told recruiters he had used cocaine in the past, and investigators later discovered that he had previously been arrested, though not prosecuted, on suspicion of smuggling after immigration officers at San Ysidro, Calif., detained him with two illegal immigrants in his car.

In March, two Border Patrol supervising agents in California, Mario Alvarez, 44, and Scott McClaren, 43, were also charged with smuggling. The agents had helped set up an antismuggling program with the Mexican authorities. They have pleaded not guilty and are awaiting trial in San Diego.

In recent years, several agents have also been convicted of assaulting border crossers and other abuses. Advocates for immigrants have long accused the agency of too often stopping people, particularly Latinos, without proper justification and of giving little public accounting of any results of abuse accusations.

"It seems like they just hired Border Patrol agents from Ohio and brought them down here and put them in our communities," said Fernando Garcia, director of the Border Network for Human Rights, a group based in El Paso that monitors law enforcement at the border in Texas and New Mexico.

Todd Fraser, a spokesman for the Border Patrol, said a relatively few rogue agents had drawn more attention than the vast majority of honorable ones, including several who had won praise inside and outside the agency for efforts to rescue immigrants stranded in the desert.

Mr. Fraser said much of the concern about agent misconduct was outdated and overblown. Agents, he said, go through increasingly extensive preparation for jobs that often involve great risks, including the threat of confrontation with armed smugglers.

"Border Patrol agents go through a long and intensive training program that makes them among the most highly trained and professional officers out there," he said.

Some critics have also expressed greater confidence in the agency. Representative Xavier Becerra, a California Democrat who in the early 1990's called for a federal commission to oversee the agency because of its many problems, said it had made great strides in raising standards and curtailing questionable tactics.

"I certainly think over the years we are seeing border enforcement become more professional," Mr. Becerra said. "They have done a lot to get in line with professional standards."

The Border Patrol has over the years had trouble keeping agents and hiring enough to compensate for the losses. The agents' union blames entry-level pay, which is $35,000 to $40,000, depending on experience, generally lower than many local and state law enforcement agencies.

The work, too, is demanding and calls for solitary patrols in the dead of night in forbidding terrain, often arresting the same people over and over again. In all, the agents are responsible for 6,000 miles of land border with Mexico and Canada and 2,000 miles of coastline around Florida and Puerto Rico.

"It is mind-numbingly boring to sit in one spot 10 hours a day and watch people stream by and be told your job is not to chase them but call the guy behind you," said T. J. Bonner, president of the National Border Patrol Council, the agents' union, referring to a common tactic of stationing agents and vehicles as a deterrent to smugglers. "The problem is there often is no guy behind you, because we are short-staffed."

A large number of agents left shortly after the Sept. 11 terrorist attacks to take better-paying jobs in the newly expanded air marshal service. Many have since returned to their old posts, however, and the patrol reports attrition has fallen to about 6 percent, after spiking to nearly 20 percent post-9/11.

To help meet recruitment goals, the agency has begun an advertising campaign that emphasizes the potential excitement of the job; has raised the maximum starting age to 40 from 37, to attract more military veterans fresh from their service; and has shortened the 20-week training course for recruits who have a command of Spanish, which all agents are required to know.

The large unknown, Mr. Bonner and others said, is whether Congress will provide the money in coming years to hire agents and whether the agency can bring in enough quality recruits to meet Mr. Bush's goals.

Although Congressional legislation authorized 2,000 more agents this year, the final budget wrangling left money for only 1,500. "It's going to be tough, and it's going to be a challenge, but we are confident we will be able to do it," said Maria Valencia, an agency spokeswoman. "But the money is the key part in all of this."

The Border Patrol traces its roots to a Texas Ranger named Jeff Milton, one of the last of the Old West gunslingers who gained fame as one of the men who helped hunt down Geronimo and patrolled the relatively newly drawn Mexican border in the 1880's with horse and pistol. A 1948 biography of him is subtitled "A Good Man With a Gun."

Its agents, some still riding horseback among the tumbleweeds, rely on an arsenal of guns and high-power weapons that surely would have awed Milton, as well as tools he could never have imagined: pilotless aerial drones, all-terrain vehicles, infrared night scopes, embedded motion sensors. These days, the job still attracts applicants with a bit of cowboy in them, people who enjoy the outdoors and do not mind the often rough-and-tumble borderlands.

Devin Harshbarger, 25, is in his first two months on the job at the Casa Grande station 50 miles southeast of here, some 700 miles from his hometown, Cheyenne, Wyo.

"After 9/11, I wanted to do my part to help keep terrorists out," Agent Harshbarger said, adding that he was also drawn to working outdoors.

The job also attracts people motivated by the immigration debate.

Adolfo Diaz, 30, an Air Force veteran who is another new recruit, said he got tired of illegal immigrants crossing his family ranch near the Arizona-Mexico border.

"Individuals have come to the house, and they have threatened neighbors and families," Agent Diaz said. He described his first arrest, of some 25 people hiking across the desert, as scary because he and the two other agents were outnumbered.

But there is debate whether the new agents can significantly ebb the flow of people crossing the Mexican border, a never-ending stream that another new recruit, Christine Treviño, called "really crazy."

Last year, with 11,106 agents, the Border Patrol arrested 1.2 million people on charges of illegally crossing into the United States; in 1995, with 4,876 agents, it arrested 1.3 million. Arrests peaked in 2000, with 1.6 million made by 9,078 agents, and have swung up and down since even as the ranks of agents has swelled. The Border Patrol estimates that 98 percent of the arrests each year are made on the Mexico border and says a majority of the people detained are Mexicans who are returned to their country, usually within hours.

The data, and the mix of political, economic and social factors that contribute to illegal immigration, make it difficult to explain the erratic nature of apprehensions and undermine "the widely accepted assumption that border security will be automatically improved by the hiring of more agents," found an analysis by the Transactional Records Access Clearinghouse, a research group connected to Syracuse University.