Initially, university doctors, suspecting an infection behind the infants’ sudden illness, ran a series of test that came back negative. Only then did they test the babies’ urine, thinking, according to one doctor, that a pathogen or agent in a heating or air-condition unit might have caused the infants’ sudden ailment.

In a twist, three of the infants placed in artificial respirators after being struck ill were given morphine as part of the standard treatment, according to doctors. But the tests showed traces of morphine in all of the infants — even the two who had not been prescribed the pain killer.

The fact that all five infants started experiencing respiratory difficulties at roughly the same time led officials to conclude that the five had been poisoned around the same time, most likely during a night shift in late December.

The morphine that is officially prescribed by doctors at the hospital in Ulm is kept locked away, and each use is carefully recorded. Still, the police believe the hospital’s morphine was used to poison the babies. According to hospital doctors, the suspect had access to the morphine storage unit on the ward.

The police were called on Jan. 17, nearly a month after the crime was committed — a delay apparently tied to the hospital staff’s search for a biological infection and the Christmas break. The police eventually interviewed all six people who were on duty during the night shift — two doctors and four nurses — and searched their personal and work areas on hospital grounds.

In the nurse’s changing locker, the police found the feeding syringe with breast milk laced with morphine. The police say they believe the nurse in Ulm gave the babies morphine during their feeding, but the investigation is continuing.

Doctors say there will be no lasting effects on the five babies’ health.

“It is only thanks to the quick and immediate actions of the hospital staff the lives of the five infants could be saved,” Bernard Weber, the head of the Ulm Police, said at the news conference.