Randall Liberty, commisssioner for Maine Department of Corrections, in January 2019. Credit: Gabor Degre / BDN

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Officials still aren’t sure how the novel coronavirus made its way into Maine’s second largest prison last month, infecting four inmates who have all since recovered, the state’s corrections chief said.

On May 19, a male prisoner in his 20s at the Maine Correctional Center in Windham tested positive for COVID-19, the illness caused by the virus, prompting state officials to test all inmates, staff and other outside workers who entered the facility.

[Our COVID-19 tracker contains the most recent information on Maine cases by county]

Many suspected the universal testing would reveal a positive case among at least one staff member or vendor, who, unlike the inmates, come and go from the penitentiary and may have carried the virus inside. Since the pandemic began, officials have severely limited admissions to the prisons from county jails to prevent the spread of the virus, and the sick prisoner had been admitted months before he fell ill. It can take up to two weeks for people to show symptoms of COVID-19 after being exposed.

But the testing only found positive cases among three more inmates, none of whom had entered the prison recently either. Meanwhile, Maine Department of Corrections officials worked with the Maine Center for Disease Control and Prevention contact tracers, hoping to solve the epidemiological puzzle by identifying the path of the virus along lines of personal contact.

“We’re not quite sure yet” how the virus got in, said Randy Liberty, commissioner of the Maine Department of Corrections, in an interview Thursday. “Every lead we followed came up negative. It still remains a mystery.”

Yesterday, the department completed a second round of testing to ensure the virus had not further spread among asymptomatic carriers and discovered no additional cases of the virus beyond the original four.