(CNN) The head of the Mississippi Department of Corrections is now asking the FBI to help investigate the deaths of 15 inmates who died in August.
Pelicia E. Hall, MDOC commissioner, said in a statement Friday that she has been in contact with the FBI and the Mississippi Department of Public Safety regarding the deaths.
Earlier this week, information was released saying 12 prisoners had died while in custody. Two more inmates were added to that list Thursday and one more was added Friday, bringing the total to 15 deaths.
"While we believe that most of the reported deaths during the month of August are from illnesses or natural causes, such as cancer and heart disease, based on available information, we are seeking assistance from others outside the department in the interest of transparency," Hall said in her statement.
Hall said Tuesday "the number of deaths the department is reporting is not out of line with the number of deaths in previous months." The latest DOC figures showed 47 inmates died in 2015, 53 died in 2014 and 71 in 2013.
Hall said in a Tuesday statement that the state's inmate population -- 19,425 as of Tuesday -- comprises "people from all walks of life and with all types of pre-existing conditions."
Reports from CNN affiliates suggest that at least one inmate did not die from natural causes.
According to The Clarion-Ledger and DOC, five of the dead prisoners were from Central Mississippi Correctional Facility, three were from the Mississippi State Penitentiary at Parchman, five were from South Mississippi Correctional Institution and one was from Wilkinson County Correctional Facility. It is unknown where one of the inmates was being held.
Of the 15 inmates, one died in prison and nine died at a hospital. It is unknown where five of the prisoners died.
The cause of death for each of the 15 inmates is pending the results of an autopsy.