Five forensic doctors are conducting the postmortems, with police and health authorities suspecting bacterial or viral infections, malfunctioning incubators or medical malpractice as possible causes.
Findings of the initial examinations are expected later Monday, while full autopsy results will require a longer period of up to one month, the NFS said.
The Korea Centers for Disease Control and Prevention, which is conducting a separate investigation into the cases, said Monday that at least three babies may have been infected with Gram-negative bacteria of some kind, although its link to their deaths requires further examination.
The three babies had their blood tested about six hours before they died and the KCDC is looking into the blood samples to determine the exact type of bacteria, it said.
Two of the four had necrotizing enterocolitis, a common surgical emergency among premature babies in which tissues in the intestine become inflamed and start to die, it also found.
The deceased were among 16 premature babies in incubators at the intensive care unit at Ewha Womans University Medical Center in western Seoul.
They died one by one between 9:31 p.m. and 10:53 p.m. on Saturday, a hospital official said during a press conference Sunday.
Several babies encountering cardiac arrests in such a short period of time is rare in the ICU, the doctors said, adding that they have no idea why it happened.
Families said the babies all had bloated stomachs and difficulty breathing before their deaths.
The Seoul Metropolitan Police Agency said it is investigating the hospital and its staff doctors, leaving all possibilities open. It has secured relevant medical records and are looking into them, it added.
Officials from the Seoul Metropolitan Government, where the hospital is located, and other health institutions are also conducting an epidemiological survey and monitoring the health conditions of other infants who were at the same intensive care unit with the deceased.
Immediately after the mysterious series of deaths, eight of the 12 remaining patients at the unit were moved to different hospitals while four were discharged.
Officials said all of them appear to be in a stable condition.
Meanwhile, Prime Minister Lee Nak-yon ordered a thorough and swift investigation to determine the cause of the babies’ deaths.
“A tragic incident that shouldn’t have happened has happened. I don’t know what to say to the parents of these babies,” Lee said during a meeting with senior aides. He also instructed officials to take steps to prevent similar incidents from happening again.
In September, the hospital came under fire over IV fluid containing an insect given to a 5-month-old baby for more than 14 hours.
By Kim Da-sol (email@example.com)