The Chester County jail has seen a rise in COVID-19 cases in recent weeks, officials said Wednesday, and one inmate is believed to have died from the virus in the first death attributed to the pandemic at the facility.

On Monday, 18 staff members and 143 inmates tested positive for the virus, according to county spokesperson Rebecca Brain. The number of inmates with the virus dropped to 84 on Tuesday after further testing, she said.

The source of the outbreak at the facility, in Pocopson Township, is under investigation, officials said. Family visitations at the prison have been temporarily suspended, and legal professionals must wear masks when entering the jail.

Melvin James Anderson IV, 52, died in the jail Dec. 7, according to county coroner Christina VandePol. Anderson, who was awaiting a criminal trial on burglary charges, had experienced respiratory symptoms consistent with COVID-19 for about two weeks before his death, but staffers there were unaware that he had contracted the virus. During his autopsy, he tested positive for COVID-19, VandePol said, adding that it likely caused his death. The official ruling is pending further study, she said.

“We were not aware at the time that there was a COVID outbreak at the jail,” VandePol said. “Obviously, we are considering this as we continue our investigation.”

One inmate remained hospitalized Wednesday, according to Brain. She declined to provide further information, citing privacy laws.

To help curb the outbreak, staff at the prison will undergo mandatory COVID-19 testing, and rapid tests will be given weekly after that, Brain said. New inmates entering the facility will be tested, quarantined for 14 days and offered a vaccine if they haven’t yet received one. Inmates will also be tested when leaving the facility for court appearances.