Charges have been dropped against a nurse accused of contaminating saline at Stockport's Stepping Hill Hospital.
Rebecca Leighton, 27, of Heaviley, was arrested by police investigating the deaths of several patients in July.
Greater Manchester Police have revealed they are investigating 40 cases of contamination including seven deaths.
Prosecutors said there had not been "sufficient evidence" for whether the case could go ahead. Miss Leighton said she had been "living in hell".
Officers said Miss Leighton was released from prison on Friday afternoon after she was told by the governor at HMP Styal in Cheshire that the charges against her were being dropped.
They said police were in contact with her to guarantee her safety and "to try and help her back into the community".
The Crown Prosecution Service said it was "no longer appropriate" to continue the case against Ms Leighton.
Solicitor Carl Richmond read a statement on behalf of Ms Leighton in which she thanked her family, friends and members of the public who "never doubted my innocence during this living nightmare".
It read: "I've been in a living hell and was locked up in prison for something I had not done.
"It was so frustrating for me knowing that the person who actually carried out these terrible acts is still out there.
"My life has been turned upside-down. All I ever wanted to do was to pursue a profession in nursing and care for my patients."
Nazir Afzal, Chief Crown Prosecutor for the North West, said Miss Leighton had been charged on the basis there was "reasonable suspicion she had committed the offences and there were reasonable grounds for believing the continuing investigation would provide further evidence within a reasonable amount of time".
He added: "When we make a decision on this basis, it would be wrong of us to keep a suspect in custody indefinitely without keeping a very close eye on what evidence is emerging and whether objections to bail can be justified.
"The inquiries, which are still ongoing, have not so far provided us with a stronger case which would meet the test that there is sufficient evidence for a realistic prospect of conviction. This is the test all cases must meet for a prosecution to go ahead."
Lawyers representing Ms Leighton said on Friday the case against her was "weak" at a bail application made on her behalf in August.
They said Mr Justice Henriques had told the court the evidence against Ms Leighton amounted to her fingerprints being on a saline bag, which had been damaged by a needle.
Although the plea for her to be given bail was rejected at that hearing, her lawyers said the judge accepted "many people" had access to the saline bags and the fluid and that Miss Leighton had every reason to be touching the items in her role as sister.
'No stone unturned'
Assistant Chief Constable Terry Sweeney defended the decision to charge Rebecca Leighton, saying it was "the correct one at the time based on the information and evidence available then".
He confirmed that officers had interviewed 200 out of 700 people who could have had access to the area concerned, including potential victims as well as hospital staff, patients and visitors.
Mr Sweeney said thousands of items were subject to examination and officers would "leave no stone unturned".
He added: "We are clear that there was contamination in a number of saline products," he said.
"It will take a number of weeks to complete the investigative process, the forensic process and most importantly the medical analysis which needs to take place."
He added that there had been no further incidents of contamination since 16 July.
The Nursing and Midwifery Council (NMC) has said that an interim order suspending Ms Leighton from its register would remain in place.
Greater Manchester Police were initially looking into the deaths of five patients at the hospital, although two were later dropped from the investigation.
Police are continuing to probe the suspicious deaths of Tracey Arden, 44, Arnold Lancaster, 71, and Derek Weaver, 83.
Four more suspected victims have yet to be named but their families are aware that the cases have been referred to police by the coroner.
Of the seven deaths, the force said there were two confirmed cases where there is a "high probability" that the deaths were caused by contaminated products.
The alarm was raised when a higher than normal number of patients were reported to have "unexplained" low blood sugar levels.
Several charges were made against Ms Leighton in July relating to the alleged tampering of saline ampoules, saline bags and medical products.
These - along with one charge of theft of medication from the hospital - have now been dropped.