(CNN) Chicago's Cook County Sheriff Tom Dart stepped in front of news cameras and reporters and said 38 of their detainees in Cook County jail had now tested positive for the novel coronavirus with over 120 still left to be tested on Friday.

"We knew this was coming," said Dart. "We knew, between people being asymptomatic and staff coming in and out every day, that this was going to happen."

The 38 confirmed Covid-19 cases announced Friday is a sharp increase from just a day prior when America's biggest single site jail had 17 confirmed cases in detainees. Saturday, that number jumped to 89, an increase that's not expected to end there with dozens still to be tested.

"Jails in this country are petri dishes. They're the government equivalent of nursing homes or cruise ships, said Toni Preckwinkle, Cook County Board president. "It's very difficult in a jail to maintain social distancing."

It's why the county has now tried to minimize the number of detainees that are at risk to begin with, screening and releasing nonviolent pretrial defendants. Of the around 5,000 people that remain in the jail, the county estimates about 80 percent of them are violent offenders, leaving around 1,000 or so that could safely be released.

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