While Ricardo learned to change diapers this fall, the mother of his two young children, Yancy Zamora, was stuck on Rikers Island. An illegal immigrant, she is now detained at a New Jersey jail, awaiting deportation to her native Costa Rica.

In August, Ms. Zamora punched a neighbor, who called 911 but said she never thought the call would lead to a deportation. “I did not want to press charges because I am a mother, and beyond that, I have my principles,” the neighbor said in Spanish.

Why Ms. Zamora assaulted her neighbor, who two months earlier had accompanied her to the hospital when she gave birth, is not entirely clear. Her husband said the woman next door dumped trash on their doorstep; the neighbor, an immigrant from El Salvador, said Ms. Zamora suffers from psychological problems.

The neighbor’s 911 call snagged Ms. Zamora in a larger dragnet. Local and national immigration agents, unable to target all of the nation’s 11 million illegal immigrants (500,000 of whom are in New York, the Pew Hispanic Center in Washington estimates), are instead going after an unsavory sector of that population: so-called criminal aliens.

Even though she was found not guilty on the assault charge, Ms. Zamora had already been labeled a criminal alien based on a separate warrant issued days before, related to her overstaying her tourist visa by more than a year. Instead of being transferred to immigration detention or released within two days after her case was adjourned, as the law requires, she was imprisoned at Rikers for an extra seven weeks.

The punishment baffles Ricardo, a 26-year-old Mexican immigrant, who requested his last name – which is different from his wife’s – not be used because of his undocumented status. He said almost everyone in their Brooklyn neighborhood, including the neighbor and her family, are illegal immigrants. True, he said, his wife punched the neighbor with such force that the woman’s lip turned blue and swelled, but he cannot understand why she, never before having committed a crime, was singled out for deportation. “Why do the judges punish the little ones who can suffer more?” he asked, speaking of their two children, ages 4 and 4 months. “Only for not having papers. She didn’t rob, she didn’t kill.”

Ricardo met Ms. Zamora five years ago in a Queens discotheque, not long after he slipped across the southwest border. Although they are both undocumented and from different countries, until Ms. Zamora was arrested he said they never considered the possibility that either could be deported. “We’ve seen many people who have been here 10 years, 20 years, and never had problems with the police,” he said.

Even the neighbor, who maintains Ms. Zamora also ripped at her hair and kneed her in the chest, said she is shocked by the deportation order.

Sitting in her meticulous apartment, the neighbor dissolved into tears recounting the turn of events. Frightened and upset by the assault, she said she had called the police for protection and was told Ms. Zamora would receive psychological help and probably a sentence of community service. The El Salvadoran said she was shocked when her children told her they saw Ricardo on a Spanish television channel pleading for help to pay the $5,000 immigration bond. “If I wasn’t poor,” she said, “I would get the money together myself for her.”

It would be welcome help. Ricardo, a day laborer from Mexico, has been scrambling ever since the arrest on August 27 to free his wife while learning how to play Mr. Mom.

“There was no milk. I didn’t have any money for clothes,” Ricardo, who has no close family in New York, recounted while sitting on the bed in the Bushwick apartment he shared with Ms. Zamora. “The children cry a lot. The older one asks every night, ‘Where is my mama? Where is my mama?'”

To make matters worse, he lost his job two days before his wife’s arrest, when police raided the pirated goods factory in Queens where he worked and shut the site down. Since then, he has taken to the streets, offering labor in anything from construction to cleaning. “I am trying to work, but I want to take care of my children,” he said, tears welling in his eyes.

When the judge adjourned Ms. Zamora’s case on September 8, granting an acquittal contemplating dismissal, Ricardo said he thought the nightmare was over. But his wife did not come home.

That she was imprisoned for more than 48 hours after the judge adjourned the case was unlawful, according to a staff attorney with the New York State Defender’s Association’s Immigrant Defense Project, Benita Jain. “There is no reason someone should have to stay jailed at Riker’s,” she said. The case, nonetheless, is not unique at Riker’s or other correctional facilities, Ms. Jain said “I think immigrants can get lost in the criminal justice system, because the federal government is using the criminal justice system much more aggressively to identify people for detention and deportation,” she said.

It is not clear why Ms. Zamora was held at Rikers for more than seven weeks. On Friday, a Correction Department spokesman, Thomas Antenen, said the agency was looking into Ms. Zamora’s case.

She was transferred from Rikers on October 27, not long after Asociacion Tepeyac, a Mexican advocacy group in Manhattan, made some calls to the facility and her legal aid lawyer.

Leaving Rikers for the Passaic County Jail in New Jersey did not improve the situation for Ms. Zamora. Immigration detention, she has told her family, has been even worse, with eight women to a room, cockroaches and rats in evidence, inedible food, and no physical contact during visiting hours. On Ms. Zamora’s 31st birthday earlier this month, her family ventured out to Patterson, N.J., to visit her and was shocked they could talk to her only through a small box with a telephone. They could not hug her like they had at Rikers. The 4-year-old was upset by his mother’s distraught response. “He kept asking her, ‘Why are you crying?'” Ricardo said.

Now, the best-case scenario, according to her pro bono lawyer, Jesus Pena, would be a congressional request to receive a deferred deportation. That would allow Ms. Zamora to disappear back into the illegal shadows of New York. If that fails and the judge sends her home, she will choose voluntary repatriation and return to Costa Rica.

Ricardo, who still has not fully thought through the likely possibility his wife will not legally be able to return to their Brooklyn home, says he cannot imagine he and their children, both American citizens, would move to Costa Rica to be with her. Mr. Pena estimated that about half of his clients sent to their native countries find their way back to America, mostly through illegal means. With her children here, one can only assume that Ms. Zamora, lawfully or not, would choose to join their ranks.