Lucy Letby was detained on Monday on suspicion of murdering eight babies and for the attempted murder of nine at the Countess of Chester Hospital.
Police have now confirmed she has been released on bail pending further inquiries.
The initial inquiry was launched by Cheshire Constabulary two years ago to look into the deaths of 17 babies and around 15 non-fatal collapses between March 2015 and July 2016.
This led to Ms Letby being arrested for the first time in July last year on suspicion of murder in relation to the deaths of eight babies and the attempted murder of another six babies.
She was later bailed, but this week was re-arrested for those same charges and arrested for the attempted murder of three additional babies.
Detective inspector Paul Hughes, who is in charge of the investigation, said: “I would like to remind people that due to the nature of this investigation it is extremely challenging and remains very much active and ongoing.
“There are no set timescales for this and we would ask people to bear with us whilst we continue with our enquiries,” he noted.
“We remain committed to carrying out a thorough investigation as soon as possible and I want to reassure people that we are doing everything we can, as quickly as we can, to identify what has led to these baby deaths and collapses,” he said.
He added: “We fully appreciate that it continues to have a big impact on all of the families involved and staff and patients at the hospital, as well as members of the public.”
Mr Hughes said parents of the babies were being kept fully updated and were being supported throughout the process by specially trained officers.
“This is an extremely difficult time for all the families and it is important to remember that at the heart of this there are a number of families seeking answers as to what happened to their children,” he said.
Police have not named the nurse, but she is understood to be Lucy Letby.
If anyone has any information that they want to pass onto the investigation team they are encouraged to get in touch via the Operation Hummingbird mailbox at [email protected].