The fire started at about 9 a.m. at the Sherry Lake Apartments at 1801 Butler Pike, authorities said.
Chopper 6 showed smoke coming from the building, as crews continued to fight the fire.
The 16-unit building was evacuated. One pet died from smoke inhalation, one civilian suffered minor injuries and was treated on the scene. One firefighter also had minor injuries.
"I got the dog, got the insurance paper box, went to the door and the firemen were already here," said Michael Owens, a resident.
Last week, the apartment complex management went through every building and replaced all the fire alarms and carbon monoxide detectors.
More than three hours after the fire started, crews were still working to fully extinguish all flames and hotspots.
Amy Brenner was able to get back into her apartment to grab what she could, including original copies of family photos from 1913. She waited anxiously in her car with her daughter, dog, and cat to see what would be left.
"...I don't know what it looks like right now. I'm dying because I saw the flames and I don't know what to do. I've never been homeless before. I have resources, but I've never dealt with an emergency like this," said Brenner, a tenant of nearly five years.
The Red Cross is assisting eight families with temporary housing.
Fire crews are still working to determine the cause of the fire.