With coronavirus cases rising in nearly all the region’s jails, prisons, and detention centers, Montgomery County officials set out last week to determine just how entrenched the disease had become behind bars and tested every inmate in their custody.

What they found was sobering, and could indicate infection rates at corrections facilities across Southeastern Pennsylvania are several times higher than what is currently being detected.

Of the 948 inmates, 177 — or roughly 18% of the county’s incarcerated population — tested positive, a rate of infection more than 30 times greater than what Montgomery County had identified before it began its mass testing over two days last week.

Perhaps more surprising, said Val Arkoosh, chairperson of the county’s board of commissioners, 171 of those positive inmates exhibited no symptoms at the time their tests were administered.

The results offer crucial new data for epidemiologists studying asymptomatic transmission and suggest that many more people than previously believed may have caught and unwittingly spread the virus, which has caused the death of nearly 1,900 Pennsylvanians so far.

“This could tell us a lot about the number of people who might have the virus but aren’t showing symptoms and may never show symptoms,” said Aimee Palumbo, an assistant professor of epidemiology at Temple University. Montgomery County “knows who is positive [now] and can monitor these people in a closed setting to see if they develop symptoms down the road, or if they’re truly asymptomatic.”

The county’s findings largely track with those of other mass testing experiments conducted in closed environments across the nation over the last month, including a prison system in Ohio and two homeless shelters in Massachusetts.

The U.S. Navy tested all 4,800 crew members of the USS Theodore Roosevelt amid a coronavirus outbreak on board. The ship’s percentage of positive cases — 12%, or 600 sailors — was slightly lower than that of the Montgomery County jail, as was its asymptomatic rate. But at 60%, the number of sailors who showed no symptoms despite positive tests was high.

Arkoosh, a physician with a background in public health, said the results of the Roosevelt testing spurred her idea to test all of the jail’s inmates and staff over 48 hours last week. At the time, the county had identified only six inmates with the coronavirus.

“I just knew there had to be more,” she said in an interview Tuesday. “What happened on the Roosevelt made it clear to me that young, healthier people could be showing no symptoms when they contract this — and that it could be a large cohort.”

What her findings mean for the population at large remains unclear. Palumbo, the Temple epidemiologist, cautioned against drawing too many broad conclusions from results at a jail, where cramped quarters make proper hygiene and implementing recommended social distancing guidelines a challenge.

It’s likely, she said, that the rate of infection among people who are not incarcerated would be much lower.

But if the roughly 18% infection rate in Montgomery County’s jail is applied to other corrections facilities across the region, a startling picture begins to emerge.

For example, a similar rate in the roughly 3,600 population of the Philadelphia jails would mean as many as 682 inmates there could be infected already. City officials had only reported 190 positive tests as of Tuesday.

In Delaware County, where the GEO Group, the for-profit company that runs the George W. Hill Correctional Facility, is currently conducting similar mass testing on its inmates and staff, the number of positive inmate cases could be as high as 196, according to an Inquirer analysis based on Montgomery County’s findings.

Epidemiologists say the growing body of evidence from mass testing experiments suggests that the coronavirus could be harmless for a much wider swath of the population than previously thought. But it also raises sobering questions about the limits of contact tracing and other tactics that public health experts have relied upon so far to control transmission.

Arkoosh has said for weeks that testing all residents and staff in group-living facilities like nursing homes and jails is the only way to catch up with the virus’ spread.

Given that 96% of those who tested positive in the Montgomery County jail were asymptomatic, yet still presumably capable of spreading the disease, Palumbo wondered what that meant for efforts to stop transmission among broader swaths of the population. Those answers become even more important as governments make decisions about how they will avoid a second wave of infections as they begin to lift stay-at-home orders and reopen their economies.

“Not many asymptomatic people are being tested at this point,” she said. “We just don’t have the capabilities yet.”

City officials in Philadelphia said Tuesday that in an ideal world, they, too, would conduct similar mass testing. But already facing a shortage of testing supplies and limited laboratory capacity, expending resources on experiments like Montgomery County’s isn’t feasible at this time, they said.

“It isn’t clear that testing all the other people who don’t have symptoms really would have a real benefit,” Health Commissioner Thomas Farley said. “Right now, we’re trying to sort of do the testing that makes the clearest sense for infection control.”

But in Montgomery County, the results are already shaping the way officials are responding to the disease.

The county has moved the 171 positive inmates into isolation wards and put an additional 500 inmates, who had close contact with those infected, into quarantine.

“At least, we hope, we’ve got the vast majority of people safely in isolation or quarantine, separated from those who are negative,” Arkoosh said. “These are 950 human beings. I think it’s our responsibility to care for those individuals as safely and humanely as we can.”

Staff writer Laura McCrystal contributed to this article.