Dawn Brown, 44, was discovered dead Monday afternoon by her husband at the bottom of her basement stairs, the Chicago Tribune reported.
An autopsy released Wednesday said the Big Rock woman died from bite marks to her neck by a 140-pound mastiff, a police spokesperson said.
Brown and her husband took in the dog from a friend about a week previous to the attack.
Dawn Brown was a full-time firefighter paramedic.
Two other dogs in the household, a boxer and a pit bull mix, are not believed to have been involved in the attack.
Police are trying to determine what triggered the attack, the newspaper said.
All the dogs are being evaluated at Kane County Animal Control to determine when they can be returned home.