The largest jail system in the United States has no buildings of its own. Instead, it’s a shadowy complex of agreements that tucks detainees in wherever there’s available space. It has swelled substantially over the past two decades, and it is likely to grow further.

This jail isn’t run by a county or state. It’s operated by the federal government’s oldest law enforcement agency. The US Marshals Service (USMS) rents beds in hundreds of incarceration facilities for tens of thousands of federal detainees every day. Many of them end up in jails and prisons run by private companies whose facilities have a track record of poor conditions and abuses.

In fact, the service locks up nearly as many inmates in private detention as the Federal Bureau of Prisons: more than 18,000 in total. This number—along with the revenue of private-prison operators—is very likely to increase under attorney general Jeff Sessions and president Donald Trump as they continue to upend reforms of the Obama years.

What makes it all the worse: When the public does get a rare look inside this largely hidden system, it becomes clear that the marshals service’s oversight of these private facilities has been woefully lacking.

Who are the marshals?

In the public imagination, federal marshals are tough dudes who escort dangerous inmates on airplanes (think “Con Air”), or chase fugitives (think “Justified” and Tommy Lee Jones hunting Harrison Ford). But that public image sells short the many roles of the marshals service, a critical element of the US criminal justice system that prides itself on being established in 1789, before any other federal law-enforcement agency.

In addition to transporting federal inmates and arresting fugitives, the agency, which employs more than 3,700 deputy marshals and investigators, provides security for federal courts, manages asset forfeiture for the Department of Justice, and handles the federal witness-protection program. It touts itself as the most “versatile” federal law-enforcement agency, sometimes serving unusual functions, such as security for education secretary Betsy DeVos.

The service’s overall mission makes it responsible for an average of 51,000 detainees daily. If you’re charged with a federal offense, the USMS becomes the agency responsible for you as you await trial. Almost half of these defendants, according to data obtained by Quartz, are booked for immigration or drug offenses. After trial, if you receive a short sentence, you usually serve out your time in the service’s detention as well. If it’s a more serious conviction, you are transferred to a federal prison.

The sprawling USMS detention apparatus serves as a sort of federal “jail,” an organization with coast-to-coast reach that is largely removed from public view. “It’s a hidden part of the criminal-justice system,” said Bethany Carson, a researcher at the Texas-based anti-privatization group Grassroots Leadership. Since the detention is more short term, “nobody tends to pay attention.”

A swelling inmate population

Over the past two-plus decades, marshals detention has more than doubled, from just under 100,000 bookings annually in the mid-1990s to just under 200,000 in 2016. The peak was 2010, with more than 221,000 people booked. Part of this rise can be explained by the overall mass-incarceration boom in the United States. Yet the fastest-growing population within USMS detention, by far, is immigration offenders, whose numbers have climbed tenfold over this period.

This increase accelerated in the mid-2000s, when the federal government was rapidly expanding the practice of charging people detained entering the country with misdemeanors, and those who re-entered with felonies. Part of this was the infamous “Operation Streamline,” an effort in border states to ramp up immigration prosecutions that was launched 2005.

Between 2013 and 2016, the number of immigration detainees within marshals’ detention — and thus the agency’s inmate population as a whole — was falling. This is likely in part because immigration prosecutions had been decreasing in recent years. In a 2016 report from the Government Accountability Office, USMS officials outline several other reasons: declining funding for federal law-enforcement, hiring freezes, and the Smart on Crime effort launched by Obama administration attorney general Eric Holder. The program directed federal prosecutors to avoid charging low-level drug offenders with crimes that carried long sentences—a widely praised move that the Sessions Department of Justice has already reversed.

We’re likely to see another change in the chart above. The two main drivers of incarceration in the federal system, which includes marshals detention, are immigration and drug offenses. These are also the two areas where the Trump administration has declared a crackdown. In fact, according to a budget request for fiscal year 2018 from the DOJ, the service is seeking funds to accommodate an anticipated rise in detainees, identifying “enhancements to border security and immigration enforcement” as the reason.

Where to put 50,000 people?

The service does not have its own detention facilities. Instead, it uses a combination of public and private ones, through different kinds of agreements. As of 2016, the marshals service held more than 9,400 detainees in federal facilities run by the Bureau of Prisons. There were nearly 24,000 detainees in state and local prisons and jails, placed through Intergovernmental Agreements (IGA), and more than 9,400 in 15 private facilities under direct contract with the service. Additionally, the agency delegates the process of contracting with private-prison companies to state or local governments. So in effect, nearly 8,600 more detainees are held in private facilities as well (there are 25 such contracts, the USMS told Quartz). In comparison, the Bureau of Prisons holds about 21,000 of its inmates in private prisons, which is about 11% of the total population.

This means that more than one-third of USMS’ detainees are in private facilities, which is a larger share of the population than in either federal or state prisons overall. (This data comes from Freedom of Information Act responses from the USMS, obtained by the privatization watchdog In The Public Interest and the ACLU, and provided to Quartz.)

“USMS uses private facilities in situations in which a district’s operational needs for detention space cannot be met by using Federal Bureau of Prisons, state or local government facilities,” the agency’s spokesperson said.

1 According to the GAO report, this is not always the case, and the marshals sometimes place detainees in private custody, even if spots in federal detention are available. The USMS says there are several reasons for this, such as placing detainees closer to the relevant court, or their medical condition.

David Turk, official historian of the USMS, says the practice dates to about 1990, “when law enforcement was facing a crisis in jail space.” While the population that needed detention was increasing, marshals were transporting detainees “in long trips from distant locations,” he told Quartz. “This put a strain on our resources, and took deputies away from other duties. It was then that the agency considered private prison facilities.”

According to one report from the DOJ’s Office of Inspector General (OIG), in 1994, private facilities held 1% of marshals detainees. By 2000, that share grew to 16%.

Private prisons have a poor track record

Private-prison companies have long been criticized by advocates for running their facilities in inhumane and unsafe ways.

“Handing control of prisons to for-profit companies it is a recipe for abuse and neglect, and that holds true regardless of who is being held in any particular facility,” said Carl Takei, staff attorney for the American Civil Liberties Union’s National Prison Project.

While the poor conditions in federal and state prisons, local jails, and immigration detention centers run by private companies are getting increased media attention, we don’t often hear about USMS detainees. Marshals detention is usually shorter than federal prison sentences, and it’s also less discussed than headline-grabbing Immigration and Customs Enforcement (ICE) detention.

But there have been some glimpses into the system, cases that usually get publicized because the service shares a facility with another agency.

In 2014, for instance, at a federal prison in Youngstown Ohio run by private prison giant CoreCivic, then known as CCA, several hundred inmates refused to leave the recreation yard in a protest, citing bad sanitary conditions, difficulties in getting health care and high food prices at the commissary, among other issues. The Bureau of Prisons ended its contract with Core Civic in 2014. The marshals did not, keeping about 600 detainees in the scandal-ridden facility. (A month after Trump’s election ICE signed a contract for the Youngstown facility as well.)

The service also shares jail space with Liberty County, Texas. In 2015, after two inmates were found dead, a report by the state found multiple shortcomings in the facility, including repeated botching of mental-health and suicide screenings. The jail is operated by the Florida-based GEO Group, another behemoth in the prison market. It acquired the jail’s previous owner, CEC, in April 2017.

The Karnes County (Texas) Correctional Center, also run by GEO, serves as an ICE detention center and as a marshals facility. In 2014, the Texas Commission on Jail Standards found that the jail was under-staffed and overcrowded, with 46 inmates in space designated for 24.

Jerry Lemaine, a one-time ICE detainee and subject of a New York Times article on immigration enforcement (paywall), told the paper his “lowest point” came at the Karnes facility. Initially, guards would refuse him utensils at meals. After he told medical staff he was depressed, he “was placed on a 10-day suicide watch in a filthy segregation unit where he did not see a psychiatrist for a week,” he told the paper. At one point he was a victim of a racist attack from other inmates, he said. Guards would let inmates establish their own “pecking order.”

This tangled web of partnerships between various law-enforcement and immigration agencies and correctional institutions creates situations where immigration detainees, or federal defendants who have not yet faced trial — innocent until proven guilty — may be incarcerated alongside convicted criminals. “Legally, people might be in different standing—some people are being held by ICE, some people are being held on criminal charges, maybe some people are being held by the county or by the state — but in practice, the experience of folks in that facility might be the same,” said Mary Small, policy director at Detention Watch Network, an immigration advocacy coalition.

The companies themselves offer a different view of their performance, in strikingly similar ways. “As a service provider to the federal government, our focus has always been and remains on providing high-quality services through private-sector solutions to meet public-sector challenges,” a spokesperson for GEO Group told Quartz. “We are proud of our longstanding record providing high-quality and cost-effective services, while treating those entrusted to our care with the respect and dignity they deserve.” Similarly, a spokesperson for CoreCivic said that the company was “proud to have provided a valuable solution to our government partners at the US Marshals Service for many years and always stand willing to provide cost-effective solutions to the challenges our government partners face.”

The marshals service, when asked about private operators’ track record, said only that it uses private detention when other options are exhausted, and takes care to monitor the facilities: “In addition to our annual quality assurance reviews of the private facilities we use, we have stipulated in the contracts that the facilities must be accredited by the American Correctional Association and the National Commission on Correctional Health Care throughout the life of the contract.”

The costs of private detention

As the Obama administration was winding down, then-deputy attorney general Sally Yates decided to begin phasing out federal use of private prisons, citing a government report that found private facilities are less safe than public ones, and that they do not save taxpayers’ money.

Her decision did not cover the US marshals service, which has not escaped these problems. The 2016 GAO report found that the service was not efficient in its use of private contractors, and had poor mechanisms for monitoring costs. Detainee populations and overall expenditures have been falling since 2012, even as the cost per prisoner increased. The agency said the hike was in part a result of “bed guarantees,” which stipulate that companies will get paid regardless whether the facility is at capacity, or is just 80% or 90% full (these guarantees are widely used across all private prison contracts). If the number of detainees falls below a forecast for a certain period, the government ends up paying for empty beds.

The broader criticism of the industry centers on its motives.”By profiting off of detention, private prison companies have the perverse incentives to make business decisions that lead to more people behind bars,” said Benjamin Davis, analyst for In the Public Interest.

Private companies are known to lobby for “tough-on-crime” laws, donating to politicians on all levels (including Trump) in order to keep their business churning.

Sometimes the ties between private companies and the agencies they work with become even cozier. Stacia Hylton, who served as the head of the agency between 2011 and 2015, worked as a consultant for GEO Group between government positions, and just over a year after she retired amid allegations of misconductat the USMS, CoreCivic announced she was joining their board.

“The marshals service has operated under the radar for years, but with more than 50,000 people in custody on an average day, they are an enormous jail system,” said Takei. “It’s not right that they have been able to avoid public scrutiny up to this point.”

A very telling inquiry

A recent audit by the Justice Department’s OIG of a 1,033-bed, CoreCivic-run USMS facility in Leavenworth, Kansas, shows the pitfalls of private facilities. It’s essentially the first time the American public has had an in-depth look at what goes on in these facilities.

The audit exposes several crucial shortcomings. For one, it points out that the service can’t prove it made sure that the bidding process for building the facility was competitive. The geographical area was so limited, that, as Takei put it “there was no other company that could’ve possibly bid on this contract.”

2 It’s not that there is that much competition in the first place—the industry is strongly dominated by two publicly traded companies — CoreCivic and GEO.

The facility was frequently understaffed, falling below the requirements of its contract. At one point, the job vacancy rate for correctional officers reached 23%. This can be dangerous—when they have to pick up extra shifts, guards become tired, irritable and inattentive. Safety is compromised, and the inmates bear the costs—by having, as the audit shows, their hour-per-day of outdoor recreation cut by nearly half.

Perhaps the most egregious element of the audit is not the private company’s behavior. It’s the lack of federal oversight. The auditor says the marshals service was “reactive,” and instead of preventing potential incidents, the agency “often became aware of incidents after they occurred.” The USMS official responsible for monitoring the facility spent less than two hours per month inside the facility.

The OIG also found that the marshals did not know that the facility was hiding the fact that the facility was installing third beds in cells meant for two people, a practice known as “triple bunking.” The auditors were concerned that the dismal monitoring in Kansas “presents risks that may extend throughout all its other contract detention facilities.”

The private company that runs the facility faced essentially zero consequences. According to its contract and the agency’s policy, the marshals could demand price reductions for failure to fill important staff positions or other shortcomings. During the span of 10 years, the service did not demand a single price reduction for any of its 15 contracted facilities. In comparison, the Bureau of Prisons, between just 2011 and 2015, imposed more than $23 million in reductions. “And there are only two possible conclusions: one is that the marshals were failing to identify and punish violations, or that for some reason private-prison companies were miraculously offering perfect performance for the marshals at a time when they were not doing that for the Bureau of Prisons,” Takei said.

On an investor call in May, Damon Hininger, president CEO of CoreCivic said that there are “clearly a few things that we could have done better.” CoreCivic said it has rectified any shortcomings and that the company was “overall very, very satisfied with the facility.”

The marshals service said that they’ve been working with the OIG to implement recommendations from the audit.

What’s next?

The number of USMS detainees is closely tied to federal policies on drug enforcement and immigration. Recent decisions from the attorney general only suggest that prosecutions will be ramping up. “The immigration prosecutions particularly are not very factually complicated cases and so, it would be possible for US attorney’s offices to basically run prosecution mills for these cases,” Takei said.

Prison companies, their stocks already on the rise since Trump’s election, are watching closely.

On the CoreCivic call in May, Hininger said that Sessions’ decision to prioritize immigration offenses “may lead to increases in average daily detaining populations over the medium to long term” and that “we believe our value proposition and operational track record positions us well.”

On investor calls for both the last quarter of 2016 and the first quarter of 2017, the leadership of GEO group told analysts they were anticipating increased detention needs. CEO George Zoley said there will be a need for more US marshals detention space as a result of the administration’s “increased and expanded approach to border security.” He noted that as a result of Trump’s executive orders there is “an escalation of capacity need for all three federal agencies.”

Sessions put it simply when directing federal prosecutors in April to prioritize immigration cases: “This is a new era. This is the Trump era.”