One Philadelphia prison has yet to report a single case of the coronavirus. But it hasn’t tested any inmates.

As the coronavirus continues to creep its way into prisons and jails, only one of the Philadelphia region’s large-scale detention facilities has yet to report a single case.

None of the roughly 1,000 inmates in the Federal Detention Center in Center City had tested positive for the virus as of Wednesday, nor had any of its 229 corrections officers or support staff.

But whether that infection-free status is the result of sheer luck, a combination of preparation and vigilance, or something else entirely remains a matter of debate.

Sean Marler, the facility’s warden, acknowledged in court filings this week that none of the inmates under his care had been tested. None have exhibited symptoms to warrant it, he said.

“This cannot and did not happen by accident,” Justice Department lawyers argued in a brief accompanying Marler’s declaration. “All of this is the product of a sustained, all-hands-on-deck campaign to prevent disease in the most challenging time prison officials have faced in our lifetime.”

Attorneys representing three of the facility’s inmates balked at that optimistic view. With the lack of testing, they said, they could just as easily argue that "nobody at FDC has tested negative for COVID-19, which is scarcely cause for celebration.”

The dispute over what lessons to draw from the prison’s lack of an outbreak of coronavirus echoes discussions happening in prisons and jails across Southeastern Pennsylvania, where inmate infection rates in some facilities have risen to more than 13 times that of the regional population.

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Last week, the Public Interest Law Center, a nonprofit legal firm, and attorneys with law firm Dilworth Paxson sought class-action status for a suit on behalf of FDC inmates, asking the court to order the potential release of prisoners with health conditions that would make them more medically vulnerable to the virus.

As in similar lawsuits filed against the Philadelphia jails and Pennsylvania immigration detention centers, the lawyers argued that implementing social distancing recommendations and effective hygiene behind bars is impossible and keeping older or ailing inmates confined under such conditions amounts to cruel and unusual punishment.

» READ MORE: Philly jails still aren’t doing enough to stop coronavirus spread, ACLU says in new suit

But the lack of a single confirmed coronavirus case at the FDC could work against them.

Compare the federal prison’s numbers nearly two months into the epidemic’s reach into the United States with those of other adult detention facilities in the region:

Philadelphia’s jails, home to the area’s largest outbreak behind bars had counted 137 confirmed inmate cases as of Wednesday. A union official said 71 corrections officers had also fallen ill.

In Bucks County, 33 county jail inmates and 19 guards have tested positive. Delaware County’s George W. Hill Correctional Facility has seen 22 inmate cases and 42 among corrections officers.

And at SCI Phoenix in Montgomery County, home to nearly 90% of the inmate infections in the state’s prison system, 24 inmates and 19 staff have contracted the disease.

» READ MORE: ‘It’s terrifying’: Corrections officers say jails aren’t doing enough to protect them as coronavirus spreads

Those numbers may seem small compared with the thousands of cases Philadelphia and its suburbs have reported in the population at large. But the 4% infection rate among inmates and guards at Delaware County’s jail, for example, is 10 times that of the county’s wider population.

Lawyers with the U.S. Attorney’s Office in Philadelphia say there are reasons to believe that the lack of a single case in the federal prison is more than just a fluke.

“We are not Pollyannas. We know the situation can change,” they wrote. “But if that happens, it will be despite the dedication of tens of thousands of BOP employees who are fighting this battle.”

The Federal Detention Center suspended all personal and most legal visits, began screening all staff for symptoms, and staggering meal and recreation times for inmates in mid-March, days before other regional jails and prisons.

Since then, guards have quarantined newly arriving inmates for 14 days and provided prisoners with masks and cleaning supplies for their cells, Marler said.

Similar measures adopted at roughly the same time did not stop outbreaks from developing at other federal prisons across the country. Still, Justice Department lawyers said, the apparent success at FDC-Philadelphia may make it one of the safest places in the region to be at the moment.

“It may be that well-run prisons, with the ability to take extraordinary measures and isolate from the outside world, provide a firmer barrier against this horrible disease,” they wrote.

Thus far, their argument has proved convincing to judges. Dozens of cases filed by individual FDC inmates seeking temporary release have been denied with judges consistently citing the lack of confirmed cases in their reasoning.

» READ MORE: Racing against a coronavirus clock, Philly courts release 200+ jail inmates in the first week of fast-track emergency hearings

There are other possible explanations for that lack of movement by the federal courts.

The Federal Detention Center, unlike most federal prisons, houses mostly pretrial and presentencing inmates. As a result, most who pose no danger to the community or risk of flight are already out on bail. Those who remain tend to have more serious records of violence.

Judges have also cited an ongoing U.S. Bureau of Prisons administrative process that is screening inmates for suitability for release into home confinement as a reason to avoid court interference.

Only one FDC inmate has been approved through that process from the FDC, though four additional referrals are pending.

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With those measures in mind, U.S. Attorney William M. McSwain said there is no legal basis for the type of broad, court-managed review that the class-action suit is seeking.

“The petitioners implore us to ‘think outside the box,’ which is just a euphemism for ignoring the law,” he said. “I can assure the public that that will never happen on my watch.”

Still, the Public Interest Law Center and its clients may have reason for hope. U.S. District Judge Anita Brody, has ordered both sides to nominate inspectors who could observe conditions at the Federal Detention Center firsthand and report back to the court.

The government has strenuously objected, arguing that the plaintiffs’ nominee — the director for infection prevention at Jefferson Health System — might actually bring the virus into the prison as a result of her work at the hospital.

As attorneys for both sides note: All it takes is a single case to start an outbreak.