The hospital system was hit with a cyberattack in February that lasted more than a month.
Lurie Children's Hospital confirmed their network was "accessed by a known criminal threat actor."
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Hospital officials did not provide information about who the criminal actor could be, but said internal and external cyberexperts were working with the FBI to find the source.
A hospital spokesperson confirmed Friday more than 791,000 people, including patients, were affected by the breach.