A decision was taken to shut down the trust's computer network in order to combat the infection. IT services were fully restored late on Wednesday, 2 November, but the downtime led to the cancellation of at least 35 operations across three hospitals in Goole, Grimsby, and Scunthorpe.
The Yorkshire and Humber Regional Cyber Crime Unit is now investigating the case.
"Officers from the regional cyber crime team and Humberside Police have been out to the trust this week and have been working closely with them to help provide protection against any future attacks," said investigating officer Detective Chief Inspector Vanessa Smith. "We are working with all relevant law enforcement agencies as our criminal investigation continues."
A spokesperson for Northern Lincolnshire and Goole NHS confirmed to ZDNet that West Yorkshire Police are now examining the incident, but wouldn't provide comment on what sort of virus or malware infected its systems -- or how it managed to breach any defences.
The Department of Health also wouldn't comment on how the computer virus infected the trust's systems, but told ZDNet that the health service will "learn lessons from this incident" and is "determined to help all health and NHS organisations improve cyber security".
"NHS Digital is taking action by extending its computer emergency response team, CareCERT, which helps reduce vulnerability to cyber attacks and helps take decisive action to reduce the impact of a data security incident, if it does occur," said a Department of Health spokesperson.