Forensic doctors conducted autopsies on the bodies of four newborn babies Monday to determine what caused them to die successively at a local university hospital in an 81-minute period over the weekend.

The prematurely born babies were in incubators at the intensive care unit for newborns at Ewha Womans University Medical Center in Mokdong, western Seoul, when they died between 9:31 p.m. and 10:53 p.m. on Saturday, police said.

Families said the babies all had bloated bellies and difficulty breathing before their deaths.

Doctors and hospital officials said they have no idea why the babies died.

On Monday, forensic doctors of the National Forensic Service began postmortems on the bodies. The examinations had been scheduled to begin at 8:30 a.m. but were delayed by a few hours as it took more time to review medical records and interview family members.

The bodies were examined in the order the babies received CPR treatment, officials said.

Five doctors were assigned to the case, with three of them jointly examining the bodies one by one and the two others continuing to review medical records, said Yang Kyoung-moo, chief of the forensic medical team at the NFS Seoul Institute.

"As it's been well known, we have a shortage of forensic doctors. But we assigned as many doctors to the case as possible because this is a case shocking to the medical society, and there is a high interest among the people," Yang said.

Possible causes could include viral or bacterial infections, malfunctioning incubators or medical malpractice. Earlier, the Korea Center for Disease Control and Prevention said that bacterial infections could have caused the deaths, citing results of blood culture tests conducted before the babies died.

Officials said it will take about a month before the full autopsy results are finalized.

The Seoul city government, the Korea Center for Disease Control and Prevention, and other health authorities are also conducting examinations on the hospital's ICU for newborns.

Prime Minister Lee Nak-yon ordered a thorough and swift investigation to determine the cause.

"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, according to his office.

"I was briefed that health authorities are investigating while leaving all possibilities open, including infectious diseases, and that bacteria detection is suspected from the blood of newborns," Lee said. "Health authorities and police should swiftly find out the exact cause."

Lee also instructed officials to take steps to prevent similar incidents from happening again. (Yonhap)