Four more healthcare professionals have been arrested on suspicion of mistreating stroke victims at a hospital in Blackpool.

The suspects were detained by Lancashire police on suspicion of “administering any poison or noxious thing” with intent to injure patients.

It takes the total number of people arrested in the investigation into allegations of mistreatment to seven.

Three nurses were arrested in November and December last year and have since been released under investigation.

The four arrested on Wednesday – a man and three women – have been suspended by the hospital since December, Lancashire police said.

In January, detectives confirmed a murder investigation was under way following the death of a woman in her 70s on the stroke unit two months earlier. This is being treated as a separate investigation.

It is not clear how many patients may have been affected, but police have carried out a number of postmortem examinations as part of the investigation.

DCI Jill Johnston of Lancashire police said: “Our investigation into allegations of mistreatment and neglect of patients at Blackpool Victoria hospital is very much ongoing.

“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 Blackpool teaching hospitals NHS foundation trust 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.”

Johnston said no timescale had been set for the investigation and asked members of the public to remain patient.

“A number of postmortem 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,” she added.

“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 HM coroner for Blackpool and Fylde as part of the investigation.”

Wendy Swift, the chief executive of the trust, said: “We can confirm four further healthcare professionals who were suspended last year following allegations of mistreatment against patients on the stroke unit have now been arrested.

“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 cooperating 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.’’