A nurse at a German hospital who spent four days in custody on suspicion of giving morphine to five infants has been released after authorities said new test results showed their initial findings were wrong.

The nurse, who has not been publicly named, was detained Wednesday after investigators found a syringe containing what appeared to be breast milk in her locker at the University Hospital in the southwestern city of Ulm. An initial test concluded that the liquid contained morphine.

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However, subsequent testing showed the syringe in the nurse’s locker did not contain any morphine, Prosecutor Christof Lehr said Tuesday. He said he called the nurse Monday and expressed his “regret” that she was detained for four days.

Lehr said 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.

Follow-up tests on the samples conducted in the neighboring state of Bavaria determined the mother's milk and the milk found in the syringe showed no trace of morphine. The solvent, however, tested positive. It's unclear how the solvent was contaminated.

Traces of morphine had been found in the urine of the five babies after all suffered breathing difficulties on Dec. 20. Hospital staff intervened quickly and officials say the infants, who were between one day and one month old, aren't expected to suffer permanent health damage.

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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.

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.

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As the investigation continues, the nurse remains a suspect in the case, along with two doctors and three other nurses who were on duty the night of the incident, AFP reported.

The Associated Press contributed to this report.