Interviewed in jail in Des Moines, Ms. Nuñez said she used Ms. Blanco’s documents — which she had purchased from a woman she did not know — in 2003 to apply for her job at Swift, but that she never used them again.

She had hoped to work at the plant for many years, she said, perhaps long enough to see her children, who range in age from 2 to 15, graduate from high school (two were born in Iowa and are American citizens).

“I was innocent when I came from Mexico,” said Ms. Nuñez, a petite, round-faced woman who said she was devastated to find herself in a criminal lock-up. “But they don’t give you a job so easily anymore. To get honest work, you need good documents.”

While Ms. Nuñez worked at the Swift plant pushing sides of pork into a saw that sliced off the fat, Ms. Blanco was in her hometown of Bakersfield leading a life teeming with trouble. She had been in rehabilitation to shake an addiction to the drug known as PCP. She had lost custody of her children to the state child welfare authorities, and then had regained it.

As a result, Ms. Blanco said she was distracted and paid little attention to letters three years ago from the Social Security Administration ordering her to report the employment income showing up under her number. She had never been close to taking a job.

“I don’t know the person, but I’m upset,” Ms. Blanco said of Ms. Nuñez. “I think she could get more benefit from me, from my identity, than I could from her. ”

A Market for Authenticity

Of 1,282 illegal immigrants detained in the Swift raids, the majority were charged with civil immigration violations and quickly deported. But federal prosecutors brought identity-theft charges against 148 of the workers.

Court papers show that the accused did not use stolen documents to loot bank accounts or credit cards, the primary crimes that identity-theft laws seek to attack. Instead, they used the birth certificates and Social Security cards to get jobs.

Still, Matthew C. Allen, the senior investigations official at Immigration and Custom Enforcement, said that 326 Americans had reported financial complications and tax liabilities from having their identities used at Swift. “The victims have suffered very real consequences,” Mr. Allen said.

At Swift, the workers had Social Security contributions and other taxes deducted from their paychecks, but they did not file tax returns. The Social Security taxes accumulated in the accounts of the real owners. Because the documents were real, Swift managers were not alerted to any irregularities by the Social Security Administration, and no charges were brought against the company in connection with the raids.

Traffickers have devised several ways to meet the demand for authentic documents. Along the Mexican border, immigration officials said, muggers and pickpockets have learned that selling stolen documents to black market vendors can be less risky than a shopping spree with a stolen credit card.

Some Americans willingly sell their documents.

In Corpus Christi, Tex., this month, seven people pleaded guilty to selling their birth certificates and Social Security cards for as little as $100 for both. In another recent case, immigration officials said, an employee of a Michigan state employment bureau sold confidential identity information from state records to illegal immigrants seeking jobs.

A significant number of documents purchased by the immigrants here belonged to Americans, like Ms. Blanco, who were born and lived in Bakersfield, 115 miles north of Los Angeles. A number of those Americans lived at one time within blocks of each other in the same Latino neighborhood in Bakersfield, though at this point there is no explanation as to how their documents wound up on the black market.

Traffickers apparently sold and resold the documents in several places. Many of the identities found in Marshalltown, including Ms. Blanco’s, had also been used by immigrant workers in Green Forest, Ark., and Milwaukee, Wis. Neither Ms. Nuñez nor Ms. Blanco has ever been to either place, they said.

Ms. Nuñez said she was reluctant to use identity documents that did not belong to her, but she said she did not know that she could be committing a federal offense, since buying documents was routine among illegal immigrants here.

Real Documents for $800

She said she first came to Iowa a decade ago, joining a sister and brother who were both longtime legal residents married to United States citizens. Despite her family ties, no legal work visa was available for Ms. Nuñez, or for many other Mexican and Central American immigrants who flocked to Marshalltown in recent years, drawn by the jobs at Swift.

Ms. Nuñez said that after her third child was born, her husband, who was also Mexican, abandoned the family. She put out the word that she needed a steady job. Friends told her that fake documents would not be good enough to apply at Swift because the company’s vetting was thorough.

Before long, a Mexican woman she did not know knocked on her door. Ms. Nuñez said she paid the woman $800 for official copies of Ms. Blanco’s birth certificate and Social Security card.

The Marshalltown Swift plant, which employs 2,220, was always hiring, and Ms. Nuñez went to work at the standard starting wage of $11.50 an hour. The work was wearing, she said, but the pay was good.

“The line moves fast, and they want the work well done,” said Ms. Nuñez, speaking Spanish (she does not speak English). “After a while, I was on top of it. I did it because I had to.”

The immigration agents who raided the plant on Dec. 12 released many of the illegal immigrants who were single parents. Ms. Nuñez was among the exceptions.

María Barajas, Ms. Nuñez’s sister who lives 20 miles from the plant, said Ms. Nuñez called her from jail, “nervous, crying, her voice was shaking.”

Mrs. Barajas, who has two sons of her own, has been taking care of Ms. Nuñez’s children. The two oldest have grown up attending Marshalltown public schools; the youngest is 2. To support them, Mrs. Barajas said she had taken a job at Swift.

At the mention of her children during an interview in Des Moines, Ms. Nuñez, hunched in a gray-and-white striped jail uniform, began to cry.

“I risked everything so they could grow up in the United States,” she said. “I’m only asking for permission to do honest work.”

Ms. Nuñez and several other immigrant women detained in the Iowa raid who have children who are American citizens say they have resolved to fight the charges against them rather than make a deal with prosecutors that would lead to their deportation with no chance of legal return.

“She’s a mother who cut my pork chops and gave Social Security a lot of money,” said Michael H. Said, a lawyer representing Ms. Nuñez. “She deserves a medal, not an indictment.”

In a similar the case, a Des Moines jury this month disagreed. Lorena Andrade Rodríguez, 34, an illegal Mexican immigrant working at Swift, was convicted of identity theft on March 7. Ms. Andrade is appealing the verdict.

“I’m not a bad person,” she said. “My record is clean. My only mistake was to do hard work in someone else’s name.”

Tracing a Name to Its Source

In Bakersfield, Ms. Blanco cast a glance around her disheveled bungalow, clogged with crates of toys and clothes, and admitted there were many ways her documents could have slipped away. She said she did not sell them.

“I mean, I’m not organized,” said Ms. Blanco, who lives on Social Security payments for a psychological disability. “I just throw stuff here, throw stuff there. Or I’m not here, stuff has been stolen. Or I moved. Most of it was that stuff got lost when I moved around.”

Ms. Blanco also said her purse had been stolen several times by one of her sons. Iowa court records show that replacements for Ms. Blanco’s Social Security card were ordered 20 times over the last decade. Ms. Blanco said she could not remember requesting all the new cards.

She said that her father was a convicted cocaine dealer and one of her sons was arrested for assault when he was 9, and now, three years later, lives in a juvenile home. She has been arrested for petty theft, assault and drug use, and was once sentenced to three months in jail.

Waving a file of wrinkled papers that she keeps in a cellophane bag, she said that Ms. Nuñez’s employment under her name was only a small part of problems she attributed to identity theft.

She said she had difficulty renewing her driver’s license because someone else using her identity had taken out a license in Arkansas. A bank where she tried to open an account told her that it already had one in her name in another state — not Iowa.

“I know that when I get ready, I’m going to get everything all filed up, and I’m going to try to take care of it,” Ms. Blanco said. “I don’t know how, but I’m going to try.”