BERLIN (AP) — A nurse at a German hospital who was arrested on suspicion of administering morphine to five young babies has been released after new tests showed that investigators’ initial findings were wrong, prosecutors said Tuesday.

The five babies at the University Hospital in the southwestern city of Ulm all suffered serious breathing problems early on Dec. 20. Hospital staff intervened quickly and officials say the children, who were between one day and one month old, aren’t expected to suffer permanent health damage.

The hospital subsequently found traces of morphine in the urine of the babies and alerted police. Investigators concluded the powerful painkiller must have been given to them during a night shift Dec. 20 and questioned medical staff who were on duty. All denied involvement.

They found a syringe containing what appeared to be breast milk in a locker at the ward belonging to one of the nurses. An initial test concluded that the liquid contained morphine, and the nurse was detained Wednesday.

Prosecutor Christof Lehr said Tuesday that both a new sample from the mother whose milk was in the syringe and a solvent with which the original sample had been mixed tested positive for morphine. Regional authorities then asked investigators in the neighboring state of Bavaria to conduct new tests on the samples; in those tests, the milk showed no trace of morphine but the solvent tested positive.

The nurse was released on Sunday evening after a test on a morphine bottle in the ward also found no traces of her DNA, Lehr said. He called her on Monday and expressed his “regret” that she spent four days in custody.