At least 38 people have tested positive for coronavirus inside New York City jails, officials said, signaling the largest outbreak in American jails since the pandemic began. For weeks, activists and health officials have feared the virus would spread rapidly throughout the country's prison system.

Jacqueline Sherman, the interim chairwoman of the New York City Board of Correction, sent a letter to state and city officials on Saturday urging them to release inmates at higher risk of dying from an infection and to "rapidly" decrease the prison population.

"Fewer people in the jails will save lives and minimize transmission among people in custody as well as staff. Failure to drastically reduce the jail population threatens to overwhelm the City jails' healthcare system as well (as) its basic operations," Sherman wrote in the letter, which was addressed to district attorneys from the city's five boroughs, the state's chief judge and the state and city corrections commissioners.

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The letter said 21 inmates, 12 Department of Corrections employees and five Central Health Services workers have contracted the virus. There are an additional 58 others being monitored for potential exposure, according to the letter.

Federal authorities also said Saturday that an inmate at a detention facility in Brooklyn had tested positive, the first case of coronavirus in the federal system.

The board said those at a higher risk of serious illness from the virus include those over 50 and those with underlying health conditions. Sherman also called on leaders to release inmates serving time for low-level offenses, like skipping curfew or failure to appear in court.

In a statement to CBS News early Sunday, the DOC said 12 staffers and 19 inmates tested positive, slightly fewer than the number cited by the board. The department said the "health and well-being of our personnel and people in custody is our top priority."

Later Sunday, Mayor Bill de Blasio said 23 inmates who are over 50 and a low-risk of reoffending are being released from the city's jail system. He said city officials were waiting on a response from the state to release others.

The mayor said city officials have begun reviewing cases of 200 other inmates with limited time remaining on their sentences on Rikers Island. A decision on how many would be released will come on Monday.

"We're all trying to make sense of a very challenging situation in an appropriate way," he said. "We are looking at each and individual case."

Last week, de Blasio said he was looking at releasing 40 inmates across the city. In her letter, Sherman called that number "far from sufficient" to protect inmates from the spread of the virus. "New York must replicate the bold and urgent action it has taken in other areas to stem the tide of COVID-19 in the jails," she said.

Ross MacDonald, the top doctor at Rikers Island, who works with elderly inmates, has urged lawmakers to release as many inmates as possible.

"A storm is coming and I know what I'll be doing when it claims my first patient," MacDonald tweeted last week. "What will you be doing? What will you have done? We have told you who is at risk. Please let as many out as you possibly can."