At one moment, she declared with simply stated conviction that she had no idea the 15-year-old girl was being held for ransom in the house where Ms. Núñez was arrested. The next, she seemed to be holding back information about the friends she was arrested with. Ms. Núñez is only 22. She grew up here, in one of the world’s most crime-infested cities. But was she just hanging out with the wrong crowd, or is she a criminal deserving decades behind bars?

With her case and others, this is what Mexico is struggling to figure out. The number of women incarcerated for federal crimes has grown by 400 percent since 2007, pushing the total female prison population past 10,000.

No one here seems to know what to make of the spike. Clearly, the rise can partly be attributed to the long reach of drug cartels, which have expanded into organized crime, and drawn in nearly everyone they can, including women.

Detained lieutenants for cartels have told the police that some act as lookouts. Other women work as drug mules, killers, or as “la gancha” (the hook), using their beauty to attract male kidnapping victims. At least one woman, Sandra Ávila Beltrán, became a major cartel leader, before her arrest in 2007 for trafficking and money laundering.

Perhaps this shouldn’t be surprising. More women are working in every aspect of the economy, “including drug trafficking,” said Guadalupe Correa-Cabrera, an assistant professor of government at the University of Texas, Brownsville. Yet, because Mexico’s justice system is so opaque, incompetent and corrupt, it is nearly impossible to know which prisoners deserve their punishment. Human rights lawyers say this is especially true for women, who are often unwittingly used by men they love. Several women at the prison, for example, said they only realized after their arrests that the cars they were caught driving had been packed full of drugs by boyfriends or brothers.

And in a society as traditional as Mexico’s — where women are responsible for virtue and order, as Octavio Paz once wrote — mere involvement in crime is enough to cause outrage and fascination. Mexicans (and Hispanic telenovela viewers in the United States) simply cannot get enough of feminized crime. One of the most popular novels in Mexico today is “Queen of the South,” which follows a Mexican woman who becomes the most powerful drug trafficker in southern Spain. When Telemundo made it a telenovela this year, it became a ratings bonanza on both sides of the border.

Many of the women at El Cereso have received similar notoriety. Eunice Ramírez, 19, is the most famous. She was arrested last November for luring men into places where they could be kidnapped, and immediately photos from her Facebook page appeared nationwide, mostly showing her in a bikini.

American border patrol agents say that they have also been catching more attractive teenagers in short skirts with drugs taped to their inner thighs.

The result, here at least, is a female prison population both gang-connected and perfume-scented. Most of the 160 women here at El Cereso — one of several prisons with women a few walls apart from male inmates — are between 18 and 26. At least a third are still awaiting trial, most are charged with drug possession or trafficking, and daily life in their shared cells looks almost juvenile. Ms. Núñez’s walls feature a poster of Disney princesses; others are decorated with heart stickers.

This is nonetheless a dangerous place. Guards separate the women, as they do the men, by gang affiliation. Conflicts are common. “Every day,” said Ms. Núñez, her neck full of bruises, “there is at least one fight.” Or worse — a week after our visit, 17 inmates, including one woman, were killed in what the authorities described as a series of gang executions with automatic rifles smuggled into the complex.

It only highlighted the wide gap between the sexy myth of feminized crime, as portrayed on the Web and in telenovelas, and the more complex reality. Ms. Ramírez in particular fails to live up to her sensational billing. In person, she is painfully shy, speaking with a heavy lisp, barely above a whisper. When we met, she was busy with her visiting 2-year-old daughter, who had her mother’s onyx eyes, and a face severely scarred by burns sustained when arsonists burned down the family’s house after Ms. Ramírez’s arrest.

Some of the women here say they turned to crime to make money to meet their children’s needs. Other mothers, maintaining their innocence, say they are racked with guilt for being forced to abandon their families. Ms. Núñez, who arrived here three months pregnant, said her biggest regret was ever becoming friends with the group involved in the kidnapping. Although several in the group, men and women, are also inmates at El Cereso, she said: “We never talk. I have no one.”

Indeed, above all else, the women seem to feel lonely and misunderstood. Karla Solorio, 26, serving time for a drug conviction, said that she often cried at night thinking of her 7-year-old son.

“I’m just a reject,” she said, dabbing at her eyes to avoid ruining her makeup. “I’m not someone who works. I’m not someone with a family. I’m just a prisoner.”

Katie Orlinsky contributed reporting.