CHESTER, Ill. (AP) — The deaths of three Illinois prisoners over three days from “probable intoxication” of an unknown substance have raised questions about drug use behind bars and whether inmates are watched closely enough.

In a report posted online on Monday, WBEZ says documents and interviews suggest lax supervision before at least one of the deaths in September of last year at Menard Correctional Center in Chester.

In one case, an administrator said her concerns about the health of a prisoner were ignored, and that the guard who was supposed to check on the inmate every 10 minutes because he was on suicide watch admitted he hadn’t done so.

The Illinois Department of Corrections told the Chicago radio station that the agency acted quickly and that staffers could not have prevented the deaths.