An inmate at the Menard Correctional Center in Chester has become the fourth to die in the state prison this month.
But the Randolph County coroner said Monday he does not believe all the deaths are related.
Randolph County Coroner Carlos J. Barbour said Monday the fourth inmate died Sept. 15 after being taken to Chester Memorial Hospital from the prison's medium security unit.
The other three inmates died over three consecutive days, Sept. 5, 6 and 7.
The death of the fourth inmate, who was not identified pending notification of the family, appeared to be self-inflicted, Barbour said.
The number of inmate deaths at the prison in such a short period of time is highly unusual, he said Monday, noting it already surpasses the yearly average.
However, Barbour said he does not believe the four deaths are related. He added he does not suspect foul play in any of the deaths but would not elaborate further.
Barbour identified the other three inmates as Kevin Curtis, 31, of Waukegan, who died Sept. 5; Edwin Freeman, 45, who died Sept. 6; and Timothy Murray, 32, who died Sept. 7.
Barbour said autopsies have been performed but the causes of death are still unknown, pending toxicology results which could take up to eight weeks to become available.
Lindsey Hess, media administrator for the Illinois Department of Corrections in Springfield, confirmed the department is investigating the deaths, but had no further comment.
The IDOC did not respond to questions as to whether there could be a possible public health emergency at the prison.
An Illinois State Police spokesman said the ISP sent a crime scene services team to Menard in the wake of the deaths, which he said is generally standard operating procedure when an inmate dies. It is not known what the crime scene services team found at Menard.
Meanwhile, the ISP is not involved in the ongoing investigation, the spokesman said.
Curtis was sentenced to 20 to 60 years after being convicted in 2017 for stabbing a friend to death in a hotel room in Waukegan, Illinois in 2013, according to the Daily Herald.
Curtis' attorney, Kevin Malia, said last week he was "shocked and saddened" to learn of Curtis's death, but added he has not heard any additional information about his cause of death.
The hometowns and backgrounds of Freeman and Murray were not available. All three inmates have been removed from IDOC's offender database on the department's website.
• Reporter Lee Filas of the Daily Herald contributed to this report.