Increasingly, American jails are built without bars, razor wire or even guards. Instead, 21st century prisons are built from data. More and more, inmates are confined not by physical buildings but by GPS monitors, radio-frequency trackers and an array of other electronic monitoring. But make no mistake, electronic monitoring can feel every bit the prison as its brick-and-mortar counterpart.

The first time I stepped into a courtroom was because of electronic monitoring, the increasingly common form of surveillance used in lieu of posting bail or being held in jail, or as part of parole. It was in 2012, and I was a third-year Harvard law student representing a client at a courthouse in inner-city Boston. I shakily approached the lectern and delivered my argument, explaining why my client should be released from jail and put on electronic monitoring while he awaited trial. Whether through persuasion or (more likely) pity, it worked.

Not only are electronic monitoring devices being used more often, but they are also being used for increasingly minor cases.

I thought I had won, but my client’s ordeal was just beginning. His first panicked call for help came a week later. Police had showed up when his location tracker failed. And then again. And again. And again. It was a constant string of false alarms, each one of which risked a return to jail. Sometimes, it was because the power on the base station was knocked out, other times because the cellular signal was being blocked by the walls of his house.

It was torture for my client, and I don’t use that term lightly. A father in his late 50s, an immigrant with no prior charges, he was constantly terrified that he was about to be sent back behind bars until his misdemeanor case stemming from a family dispute was resolved. The day they finally took the electronic shackle off, he looked more relieved than when he was first let out of jail.

Experiences like my client’s are increasingly the norm, not the exception. At a moment when jail and prison populations are dropping, falling by 130,000 inmates since a peak in 2009, electronic monitoring is quickly expanding. Sadly, tools that began with the promise of releasing Americans from jail are now shackling many who would otherwise have been set free. And they are doing more than just tracking their movements: They are draining their bank accounts, compromising their health and even spying on them, which is why more safeguards against electronic monitoring abuses are urgently needed.

In 2005, Pew found that 53,000 Americans wore electronic monitoring devices. By 2015, the group found the number had grown to 125,000. There hasn’t been an official updated nationwide count since, but opponents of electronic monitoring believe it’s continued to soar.

The growth can also be seen in balance sheets: Electronic monitoring is one of the fastest-expanding sectors in the private prison industry. Take, for example, the Florida-based GEO Group, which reports that it alone is responsible for providing electronic monitoring devices for 100,00 people per year. Without any national guidelines or consensus, most police, probation and parole departments can set their own guidelines and choose their own brands of monitoring device, with various feature options.

Not only are electronic monitoring devices being used more often, but they are also being used for increasingly minor cases, including misdemeanors and traffic violations. So rather than simply replacing confinement, electronic monitoring is becoming the norm for many defendants who would have previously been released into probation, on bail or even on their own recognizance. This is likely part of why electronic monitoring seems to be growing faster than the reduction in the prison and jail population.

And electronic monitoring isn’t just expanding in the criminal justice system; it’s also being used to keep track of increasing numbers of undocumented immigrants. U.S. Immigration and Customs Enforcement requested an additional $30 million to add 20,000 tracking units in 2020. Private immigration bond programs, meanwhile, subject thousands more to electronic monitoring even when not required by ICE as a stipulation for receiving their bond.

Electronic monitoring compromises users’ privacy in many ways beyond tracking their location. One widely used electronic monitoring anklet has a feature that can record calls and conversations without users’ consent or knowledge. These devices raise profound Fourth and Fifth Amendment concerns when activated in this way, as they transform wearers into walking wiretaps — not only for capturing their own statements, but potentially those of friends and family nearby. In Chicago, a jurisdiction that consistently promotes electronic monitoring, these devices are reportedly being used to record children.

Electronic monitoring inflicts a high price that goes beyond constitutional concerns. Americans are not only being forced to wear government tracking devices in record numbers, they are also often forced to pay for the privilege, on top of their bail. One driver who was arrested after failing to signal reported being forced to pay $300 a month for a device rental fee on top of a $179.50 setup charge. The first month’s costs added up to more than half of his $900-a-month disability check. For many, electronic monitoring can become a debt trap.

The first electronic monitors to come on the scene a half-century ago took the form of bulky, single-purpose devices, such as anklets that secured a location transponder to the wearer’s leg with a “tamper-proof” band. These devices, similar varieties of which are still in use, have caused an array of medical complications, including everything from cuts and bruises and impaired circulation to electric shocks, hair loss, headaches and difficulty breathing. These devices are also highly visible, opening up wearers to discrimination by employers, law enforcement and members of the public. More than 1 in 5 of those on electronic monitoring report being fired or suspended from at least one job as a result.

Some firms are now turning to a new type of electronic surveillance device: smartphones. These firms sell apps and software that employ the same sort of location tracking that Silicon Valley has used for years to follow our movements and target us with ads.

The problems posed by electronic monitoring will only increase as the technology becomes cheaper and more powerful.

While some believe these systems remove the stigma of highly visible tracking devices, others are fearful about unintended privacy impacts. For example, many of these apps record users’ biometric information, and the companies that develop them don’t generally provide details on how that data is saved or shared with law enforcement. Already, a majority of states use some form of smartphone tracking, and the number of apps and the variety of tracking tools will likely only grow in the coming years.

The problems posed by electronic monitoring will only increase as the technology becomes cheaper and more powerful. If we let the movement to reform prisons become a movement for virtual prisons, the consequences could be dire for those involved in the criminal justice and immigration enforcement systems. The solution at this point isn’t a complete ban — not while it remains the only way out of confinement for so many — but we can’t allow unregulated growth to continue.