Several months ago, as a federal judge worked through a docket of smuggling cases in the bustling border city of Laredo, Texas, three people were escorted into the courtroom.

Because they were undocumented immigrants, the judge explained, they would be sent to jail. But they were not being charged with a crime. Instead, they would be compelled to testify against the people accused of helping them enter the United States.

The hearing took less than five minutes. The immigrants never spoke, not to ask questions or explain why they had made the illegal journey across the Rio Grande. They were then taken to jail, where they joined a long list of people — nearly 104,000 since 2003 — detained as so-called material witnesses in federal criminal proceedings.

While the law allowing the detention of witnesses in criminal cases dates back to George Washington’s presidency, its modern use has been most prevalent along the Mexican border as successive administrations have prioritized the prosecution of human-smuggling cases, according to an analysis by The New York Times of U.S. Marshals Service data obtained through a public records request.

The annual number of detainees first spiked during the George W. Bush administration, peaked at more than 8,500 during Donald J. Trump’s presidency, dipped with the onset of the pandemic and then rebounded last year under President Biden, with nearly 5,000 people jailed, the data show. Detentions in the first four months of 2023 were up 30 percent compared with last year.

Despite their significant numbers and sometimes lengthy detentions — nearly 850 people detained along the border in the past decade have been confined 180 days or longer — the witnesses register as little more than a footnote in the rancorous debate over unauthorized immigrants and serve as essential but unseen players in smuggling prosecutions.

Although lengthy detentions of material witnesses “deprives them of due process,” said Kenneth Magidson, U.S. attorney for the Southern District of Texas during the Obama administration, people “entering the country illegally have to face certain consequences — and this is one of them.”

“You could be a material witness, and you can get deported, or you can go to jail,” he said.

The Justice Department, in a statement, said the detainees “provide indispensable testimony” and are “held only as long as necessary to ensure a just resolution of such cases.”

But as immigration-related crimes have come to dominate federal court dockets — 56 percent of federal convictions over the past five years involved immigration offenses — the Justice Department and federal courts have developed no uniform approach to handling the witnesses, creating a jumble of local rules, standing orders and unwritten norms along the border.

Over the past decade, border towns in southeastern Texas have held more of the witnesses — and held them longer, on average — than anywhere else in the country, the Marshals’ data shows. The material-witness law in that region — part of the federal Southern District of Texas — has been administered haphazardly, often with little regard for due process, a review of the detentions by The Times found.

In some parts of the district, which stretches from Laredo to Houston, federal judges have flouted a part of the law that requires them to schedule prompt witness testimony, often by videotape, and quickly release witnesses from detention. In many instances, court-appointed lawyers have encouraged detainees to waive hearings where they could make a case for their release, and judges have regularly held them well beyond the 30- and 45-day limits instituted in other federal courts along the border.

In Laredo, that means sitting in jail for 13 weeks on average.

“They leave you there, and they abandon you,” said Yordy Emmanuel Larios-Vargas, a Mexican immigrant who was in custody for more than eight months last year.

Often, witnesses are jailed without ever giving testimony. About 1 percent of the more than 30,000 human-smuggling cases over the past decade went to trial, according to federal court data, because a vast majority of the accused pleaded guilty. And since it is not uncommon for the defendants to be U.S. citizens, they are often released on bond or under other conditions set by the court, even as the witnesses remain in custody.

The Department of Homeland Security does not track which immigrants are held as witnesses, so the agency has no data on how many go on to seek asylum after their release. Under immigration law, the witnesses are allowed to make an asylum claim. But often, they and their representatives say, those intending to seek asylum do not, because the process is confusing.

For those who return home, many face the prospect of confronting the very smugglers they were forced to testify against. An official at the Mexican consulate in Laredo described testifying as “high risk,” adding that “organized crime groups know they are contributing to the case.”

In other criminal prosecutions, including those involving human trafficking, the government sometimes offers special visas to foreigners who assist law enforcement. But material witnesses in smuggling cases are rarely eligible for this benefit and are often viewed as having committed a crime, even if they are not charged with one.

When immigrants are prosecuted for illegally entering the country, they often receive light sentences, like time served. Under the Biden administration, some prosecutors on the border rarely charge first-time offenders, who make up the bulk of the witnesses because their clean record plays well with juries.

“The good thing is that you’re not charged with a crime,” said Julio Garcia, a material-witness lawyer in Laredo who represented Mr. Larios-Vargas. “The bad thing is that you’ll probably spend more time in custody than if you were.”

Even without being charged, the witnesses are treated no differently from criminals. They wear the same uniform and must be hand- and ankle-cuffed when in transit, said Sheriff Anthony Zertuche, who oversees the LaSalle County Regional Detention Center, where a vast majority of Laredo’s material witnesses are held.

A 26-year-old Guatemalan woman was jailed near McAllen as a witness in 2014 when she was five months pregnant, according to court documents and case notes shared with The Times by her immigration lawyer.

She gave birth while in custody, bound with zip ties and with U.S. Marshals standing watch in the delivery room. Two days later, she left the baby with the doctor at the hospital as the agents led her back to jail. “She cried inconsolably,” according to her lawyer’s notes, reuniting with the baby only two months later.

Ultimately, the smuggler in her case pleaded guilty, so she never gave testimony and was released after 180 days in detention — the same length of time as her smuggler’s sentence.

An Old Law Has New Purpose

The authority to detain material witnesses was approved during the first session of Congress, in 1789, and amended by bail reform acts, most recently in 1984. It is now contained in the part of the U.S. Code devoted to criminal procedure.

For most of its existence, the law has been applied in prosecutions where witnesses might have been participants or victims in the crimes, or were afraid of the defendants — instances of gang violence, for example, organized crime or sexual assault. The law attracted rare public attention when it was invoked to detain suspects in terrorism investigations after the Sept. 11 attacks, which critics said was an abuse.

In the past 10 years, the law has been used almost exclusively to help prosecute human-smuggling cases along the Mexican border. Nearly 20,000 witnesses were held in the Southern District of Texas, the Marshals’ data shows, compared with just 14 in the Southern District of New York, which includes parts of New York City and some neighboring counties, where human smuggling cases are unusual.

John Scalia, a senior statistician at the U.S. Marshals Service, said he had a “high degree of certainty” that all the witnesses detained along the border were connected to human smuggling, though the agency does not formally track such information.

Material witnesses often end up boxed in by competing government priorities.

Judges, prosecutors and defense lawyers may want to keep them in the country long enough to testify, while immigration authorities may deport them before the criminal case is over. Prosecutors fear that the loss of witnesses — or even the use of video depositions — could weaken cases, because defendants have a constitutional right to confront people who testify against them. Just having witnesses on hand helps secure guilty pleas from smugglers.

Ryan Patrick, the U.S. attorney for the Southern District of Texas during the Trump administration, said it was hard to overstate the value of the witnesses, who often provide the only evidence in smuggling cases.

“The material witnesses are important because not only can I make a case directly against the driver of the truck, but they often can also give very valuable information leading backward into stash houses, how they got across the border, who is facilitating their crossing,” he said.

Over the years, rulings by federal courts, including the Supreme Court, have upheld the statute. The Justice Department’s Office of the Inspector General, after a 2014 review of the statute’s use after the 2001 terrorist attacks, acknowledged in a summary of its findings that the law was both “powerful and sometimes controversial” because it conferred the extraordinary ability to detain someone indefinitely, “even though there may not be sufficient evidence to support charging the person with a crime.”

Efforts in recent years to make the detention process less onerous include prerecorded testimony and the release of witnesses to relatives while they await trial. But the changes have been piecemeal.

In New Mexico, nearly all material witnesses are routinely released to stay with a family member or at a halfway house, said Barbara Mandel, the senior federal public defender in Las Cruces. Kenneth J. Gonzales, a federal district judge in New Mexico who previously served as a prosecutor, said released witnesses typically showed up for trial.

In Laredo, there is little time for individual considerations because of the volume of immigration cases. The Southern District of Texas detained just over a third of all material witnesses nationally since 2013, but accounted for 85 percent of those held for at least 90 days.

“Systems are overwhelmed, overloaded, triaging constantly,” said David Donatti, a lawyer with the American Civil Liberties Union, who in 2020 sued the warden of the LaSalle detention center, arguing that the material witnesses had been jailed without due process. The suit was dismissed.

Lawyers say judges ultimately make the decisions that keep witnesses detained, but the chief judge for the Southern District of Texas, Randy Crane, said in an interview that the power to change the system lies with the Justice Department. “We push as hard as we can, but it’s the executive branch that is the primary determinant of how long somebody is in custody,” he said.

‘I Wanted To Take My Own Life’

Days after Nohemy, fleeing gang violence in El Salvador, crossed the border last year, federal agents in Texas jailed her as a material witness. While in custody near McAllen, she encountered her smuggler, who threatened to kill her if she spoke to the authorities about him.

As soon as she got access to a telephone, she called her husband in a panic. “Don’t tell anyone where I am,” she recalled warning him. “Tell anyone who asks you that you don’t know anything about my whereabouts.”

Nohemy, 30, asked to be identified only by her middle name because of fears for her safety. The story of her journey from El Salvador, and her treatment there and in the United States, was corroborated through court documents, U.S. Marshals data and interviews with her husband and her immigration lawyer.

Nohemy said she never wanted to leave El Salvador, where she grew up, married and sent her three children to a school down the street. It had been a quiet, peaceful life until 2019, when a local gang began menacing her family after her oldest son, then 9, witnessed a murder.

Her husband and son left first. She and her two younger children waited to see if her husband got asylum, which would allow them to legally join him in the United States. For a while the gang activity disappeared and she was hopeful.

But early last year, the problems returned. One gang member sexually harassed her 10-year-old daughter, she said, and another threatened to kill her 7-year-old son. One night in March, men assaulted Nohemy outside her home.

She felt she could no longer wait and decided to make the trek to the United States illegally.

In custody, Nohemy said, she heard from her court-appointed lawyer only once, on her first day in jail — a complaint of inattentiveness that was echoed in interviews with other material witnesses.

“I was asking the officers if there’s a process, if they could inform me,” she said. “They would say they didn’t know anything.”

As time passed, she grew increasingly desperate. “I wanted to take my own life,” she said.

Ultimately, the smuggler pleaded guilty and Nohemy was not required to testify. Soon after her release, on the advice of her husband’s immigration lawyer, Julia Toro, she joined him in another state, where he had a pending asylum case.

Many immigrants who are detained don’t get such an opportunity because their access to lawyers and other resources is limited, said Jennifer Ibañez Whitlock, of the American Immigration Lawyers Association. Most witnesses’ immigration cases are paused while they are in the criminal justice system, and resume only after they are released into immigration custody.

Those in power acknowledge the frustrations with the system. Mr. Patrick, the U.S. attorney under Mr. Trump, said that “everyone involved is trying to do the best they can with the resources that are there.”

Does the system work?

“I don’t think there is a right answer, other than it is the system that we have,” Mr. Patrick said.