FRANKFURT AM MAIN (AFP) - Fresh questions emerged on Tuesday (Feb 4) in the mysterious case of five newborn babies who were drugged with morphine at a German hospital, after police said they made "a mistake" when they arrested a nurse on suspicion of attempted manslaughter.

The five babies, aged between one day and five weeks at the time, all survived the attempted poisoning on Dec 20 and are not expected to suffer lasting harm.

The nurse was detained on Wednesday after investigators searching her locker at Ulm University Hospital discovered a feeding syringe containing breast milk and traces of what initial testing determined was morphine.

But Ulm prosecutor Christof Lehr told reporters that the first test was now known to be wrong, after further analysis showed the syringe did not contain morphine after all.

The woman was released from custody on Sunday, with an apology from the prosecutor.

The decision to act based on the preliminary test result, which had not been checked against a control sample, "was in hindsight a mistake", said Ralf Michelfelder, head of the state police of Baden-Wuerttemberg, at a press conference.

The error became clear after the mother whose breast milk was in the syringe volunteered to give a control sample, which also inexplicably tested positive for the heavy painkiller.

The lab in Baden-Wuerttemberg carrying out the analysis then discovered it was their own solvent used in the tests that had been contaminated with a tiny amount of morphine.

Follow-up tests by a lab in the neighbouring state of Bavaria confirmed that neither the syringe nor the control sample contained any morphine.

"I'm very sorry for the woman in question," Lehr said. But given the urgent need to keep infants at the hospital safe, he said he had had to make a quick decision.

NIGHT-SHIFT STAFF

The nurse remains a suspect in the case, however, along with two doctors and three other nurses who were on duty that night.

"There remains an initial suspicion against these six people because of their close proximity to the infants at the time of the act," Michael Bischofberger, a spokesman for the Ulm prosecutor's office, told AFP.

The investigation is continuing "in all directions", he said.

The Dec 20 incident saw all five babies, some of them born prematurely, develop breathing problems at roughly the same time.

It was only thanks to "the immediate action taken by the staff" that the babies' lives were saved, Lehr said.

Ulm University Hospital initially suspected the infants had caught an infection.

But this was ruled out by urine tests whose results came back on Jan 16.

The tests did however show traces of morphine - although none of the infants had been due to receive the drug at that particular time.

The hospital notified the police the following day.

Often administered to treat severe pain, morphine is also used to treat withdrawal symptoms in babies born to drug-addicted mothers.

A morphine overdose can lead to life-threatening respiratory failure.