The toddler suffered serious facial injuries when she was mauled by the Staffordshire bull terrier and has undergone surgery.
The attack happened at a first floor flat in Borrowdale Avenue, Shadsworth, on Friday afternoon.
The girl was with her mother visiting a family friend when the friend's dog turned on her, police said.
The child is being treated at the Royal Manchester Children's Hospital, Pendlebury, after undergoing surgery.
Decision on charges
Her condition is said to "stable but poorly".
Det Sgt Jonathon Holmes, of Lancashire Police, said forensic examinations have been carried out at the flat.
He said officers were reviewing the information they had and a decision on whether a criminal offence had been committed would be made in the next few days.
The dog, which has no history of aggressive behaviour, is expected to be put down this weekend with the "full support" of its owners, he added.
Officers are yet to make a final decision on whether a criminal offence has been committed, but no arrests are likely.
It is expected the attack will be treated as a "tragic accident", a police spokesman said.
Staffordshire bull terriers are not a breed subject to restrictions under the Dangerous Dogs Act.
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