The King County Jail in downtown Seattle experienced its first coronavirus outbreak believed to have originated inside the facility on Sunday, sending 16 inmates into medical isolation, according to the Department of Adult and Juvenile Detention (DAJD).

An inmate who had tested negative for the coronavirus when first booked into the jail more than a month ago reported having flu-like symptoms on Sunday, says a news release issued Monday by DAJD. That person tested positive for the virus.

Jail staff then tested 69 other inmates held in the same area of the jail, and 15 of them also tested positive, despite being asymptomatic, the news release says. All 16 inmates with positive test results were transferred to the Maleng Regional Justice Center (RJC) in Kent and are in medical isolation, along with four inmates who had previously tested positive, bringing the current total to 20, according to DAJD.

In April, DAJD transferred adult inmates who were considered most at risk for complications from COVID-19 — those who are 60 or older or have underlying health conditions — into a designated housing unit at the RJC.

The 54 inmates who tested negative for the coronavirus on Sunday but were housed in the same area of the King County Jail as the 16 infected inmates have been placed into quarantine, according to the Monday news release. Jail officials have also undertaken “aggressive decontamination measures” of the affected housing units and hallways, elevators and transport vehicles those inmates may have used; they’ve also initiated contact tracing to identify any in-custody inmates, police officers, jail staff, attorneys or visitors who may also have been exposed, the news release says.

Prior to Sunday’s outbreak, the two jails had reported 47 confirmed coronavirus cases since the pandemic began, according to DAJD.

There were 1,414 inmates in the two adult jails as of Monday, about 26% fewer than the 1,899 inmates booked into the two facilities as of March 13 at the start of the pandemic, according to DAJD’s COVID-19 dashboard. Though one youth previously tested positive, there are currently no confirmed cases among the 23 juveniles now being held in juvenile detention.

Before Sunday’s outbreak, DAJD spokesperson Noah Haglund said positive test results were determined at the time an inmate was booked. Starting in October, DAJD has tested everyone booked into the two jails for the coronavirus and placed anyone who has refused a test into quarantine.