Police have this afternoon released a further update on the poisoning probe at Blackpool Victoria Hospital.
The force said a man and three women arrested yesterday on suspicion of poisoning patients on the stroke unit - one healthcare assistant and three nurses - have been released under investigation pending further enquiries.
The total number of medics arrested as part of the major investigation, which was triggered by a hospital whistle-blower in November, is now seven.
A female nurse was held in November, and a male and female nurse in December. They were also released under investigation.
All seven suspects have been suspended.
Det Ch Insp Jill Johnston, from Lancashire Police, yesterday said the force's complex investigation into "allegations of mistreatment and neglect of patients ... is very much on-going".
She said: "I understand further arrests could be concerning but I would like to reassure people we have a dedicated team of detectives working on this investigation.
“Our priority and the priority of the Blackpool Teaching Hospitals NHS Foundation Trust [which runs the hospital] is to ensure the safety of patients.
“Given the number of arrests and the nature of the allegations, it remains a complex and sensitive inquiry.
“No timescales have been set and I would ask people to remain patient while we continue to conduct our enquiries.
“A number of post-mortem examinations have taken place in connection with our investigation and we are supporting those families who have had loved ones identified as potentially being involved.
"They are being kept fully updated and supported throughout the process by specially trained officers.
“We are working closely with the trust as well as [the] coroner for Blackpool and Fylde as part of the investigation.”
Detectives are looking into allegations patients were given prescription-only sleeping pills to keep them heavily sedated throughout the night, it is understood.
The drug, Zopiclone, comes in pill or liquid form. It is widely used and, when used properly, is safe.
A number of funerals have been delayed to allow for post-mortem examinations to be carried out, families have said in recent months, though police have not said how many, if any, confirmed poisoning victims have been identified.
One of the post-mortems triggered a murder probe in January, with a suspicious injury being found on the body of Valerie Kneale, 75, who died on the stroke unit on November 16.
The resort grandmother's death is being investigated as a separate enquiry, police said, with no arrests being made.
In a statement, Mrs Kneale's family said: "We ... are completely and utterly devastated and heartbroken to have lost Val in these circumstances.
“She was the most precious wife, mother and grandmother, our constant friend and problem solver who we will miss dearly."
Wendy Swift, the outgoing chief executive at the Vic, said: “We take all allegations of this type extremely seriously and when the concerns were raised last year about the individuals they were immediately suspended.
“The trust contacted police as soon the allegations were made, and we are continuing to work with them and co-operating fully with their investigation.
“We would like to reassure our patients that all necessary actions are being taken. The hospital remains a safe and caring environment for patients and the provision of services will continue as usual.’’