Det Ch Insp Neil Third said: “Over recent weeks we have been continuing to gather as much information as we can, working with HM Coroner, to establish a cause of death.
“To enable us to take a full account and recover any available evidence, while providing legal protection to the person, we made an arrest as part of our inquiries.
“The arrest was also made to establish where there were any actions or omissions by that individual that were believed to be grossly negligent and we have now reached a position where we do not believe that to be the case.”
Emergency services were first called to the home on Ulwell Road at 05:20 BST.
About 40 residents have been moved to temporary accommodation in Dorset, Hampshire and West Sussex. The home remains empty.
A spokesperson for Gainsborough Care Home previously said they were “deeply saddened” and were “fully co-operating” with lead agencies.
