GREENCASTLE, Ind. (AP) — Officials at an Indiana prison where an inmate died last week after developing a severe flu-like illness say more than two dozen inmates are now hospitalized with similar symptoms.

Monday’s statement from the Putnamville Correctional Facility says 14 inmates are hospitalized at Terre Haute Regional Hospital and four are in intensive care.

Another 11 inmates are under observation in the prison’s hospital.

Indiana Department of Correction spokesman Isaac Randolph says officials aren’t sure what’s making the inmates sick.

The inmates developed viral-like symptoms last week that progressed in some of them to pneumonia-like symptoms.

Staff at the medium-security prison about 35 miles west of Indianapolis are replacing air-filters and taking other steps to prevent the illness’ spread.

Federal health officials are helping Indiana health officials investigate the inmates’ illnesses.