Police said Friday that blood poisoning resulting from bacterial infection is what caused four infants to die over a two-hour period at a hospital in Seoul last month.

Investigators at the Seoul Metropolitan Police Agency announced on Friday that the National Forensic Service carried out autopsies on the infants and concluded that the babies died due to septicemia resulting from an infection caused by bacteria called Citrobacter freundii.

The forensic agency found traces of the bacteria in the blood of all four infants. 

The agency said it believes that the bacterial infection was likely caused by contaminated injection equipment, medication or during the injection process. 

With the latest finding, police book five employees of the Ewha Womans University Mokdong Hospital over suspected involuntary manslaughter. The five include two doctors who were in charge of treating the infants, two nurses who violated infection control rules and the head nurse in charge of the two nurses. 

Police said they plan on summoning for questioning Cho Su-jin, head of the Ewha Womans University Mokdong Hospital’s neonatal intensive care unit, at 1 p.m. next Tuesday.

Citrobacter freundii is a common bacterium that lives in the digestive tract of healthy adults, but can cause respiratory, urinary and blood infections in people with compromised immune systems.