At least 17 people were poisoned by the contamination of saline solution at the Stepping Hill hospital last year.
Among that number are the cases of Tracey Arden, 44, Arnold Lancaster, 71, and Alfred Derek Weaver, 83, who all died.
Here are the key dates in the police investigation into the alleged sabotage at the hospital in Stockport.
:: July 7 2011 - Mother-of-two Tracey Arden, 44, a multiple sclerosis sufferer, dies two hours after her parents leave her bedside believing she was responding well to treatment.
:: July 11 - Retired photographer Arnold Lancaster, 71, dies at the hospital.
:: July 12 - Hospital staff contact police after they find contaminated vials of saline and notice "unexplained" low blood sugar levels in patients.
:: July 14 - Lung cancer sufferer George Keep, 84, and Vera Pearson, 84, die at the hospital. Police announce a week later that Mrs Pearson is part of the inquiry after her death was referred to them by the coroner because of low blood sugar levels.
:: July 15 - Police say they are dealing with a "fast-moving investigation", stating that tampering with saline solution has clearly taken place. Eleven other patients were said to have been affected but not seriously harmed. A batch of 36 saline ampoules in a storeroom near Ward A1 is thought to have been contaminated.
:: July 19 - Stockport Coroner John Pollard opens inquests into the deaths of Tracey Arden, Arnold Lancaster and George Keep as a team of 60 detectives from Greater Manchester Police (GMP) work on the case.
:: July 20 - Nurse Rebecca Leighton, 27, is arrested at the flat she shares with her fiance on suspicion of murder.
:: July 21 - Alfred Weaver, known as Derek, 83, dies at Stepping Hill. The widower was admitted with breathing difficulties 11 days earlier and the following day became critically ill after he suffered a hypoglycaemic episode. Assistant Chief Constable Ian Hopkins says the focus remains on preventing further harm.
:: July 22 - Leighton is charged with three counts of causing criminal damage intending to endanger life and three counts of criminal damage being reckless as to whether life would be endangered. She is also accused of one count of theft of medicine.
:: July 23 - Leighton is remanded in custody at Manchester Magistrates' Court.
:: July 28 - Police announce they are no longer investigating the death of George Keep.
:: August 1 - A trial date of February 28 2012 is set at Manchester Crown Court as Leighton appears via videolink.
:: August 2 - The Nursing and Midwifery Council suspends Leighton from its register pending the trial.
:: August 5 - Leighton is refused bail at Manchester Crown Court "for her own protection".
:: August 11 - Police say the death of Vera Pearson no longer forms part of the inquiry.
:: September 2 - Rebecca Leighton is freed from Styal Prison as the Crown Prosecution Service says the case against her has been discontinued due to insufficient evidence. She later tells reporters she has been "living in hell", locked up for something she had not done.
The Nursing and Midwifery Council states the interim suspension of Leighton will remain while it carries out its own investigation.
Greater Manchester Police reveal they are now looking at four additional deaths to those of Tracey Arden, Arnold Lancaster and Derek Weaver with 40 potential victims in total. The force says it has interviewed 200 out of 700 people they want to speak to, including potential victims, staff, patients and visitors.
:: September 4 - Rebecca Leighton's solicitor Carl Richmond says his client was made a "scapegoat", that police "jumped the gun" and that she is considering suing GMP.
:: September 12 - Police say they are no longer treating the four additional deaths - announced on September 2 - as suspicious. They say Tracey Arden, Arnold Lancaster and Derek Weaver were all unlawfully administered insulin but say it may take a number of weeks to establish the cause of their deaths.
In an unrelated investigation, police also attend the hospital after reports a bottle of milk may have been contaminated with bleach.
:: September 14 - A Nursing and Midwifery Council (NMC) panel hears Rebecca Leighton admitted stealing opiate-based drugs from the hospital. She said they were for her own use to treat a throat infection. The regulatory body says she can return to nursing under certain conditions if her employers agree to them.
:: September 21 - Police sources say a "Cracker"-style criminal profiler was brought in by police to help identify the mystery poisoner.
:: October 17 - Officers say 17 patients were poisoned at the hospital.
:: December 2 - Rebecca Leighton is sacked from her job following an internal disciplinary hearing at Stepping Hill into the drug thefts.
:: December 28 - Sources confirm the nurse, now 28, has launched an appeal against her dismissal.
:: December 31 - The death of an 82-year-old man is also now being investigated as his medicines were contaminated.
:: January 5 2012 - Detectives launch a new inquiry and arrest a 46-year-old male nurse on suspicion of unlawfully administering a noxious substance or poison, after days earlier a male patient's medical records were tampered with and he was given medicine he should not have received. The patient is monitored then discharged.