The Chicago police bomb and arson squad is investigating the deadly fire that also injured two other people.
The blaze broke out early Saturday morning at 3912 West Argyle in the Albany Park neighborhood.
Roxana Ocampo was five months pregnant when she died. Smoke and flames were a deadly combination Saturday for the mother and her young daughter. Ocampo called 7-year-old Itzel her "little doll" and "her treasure." They died together in their second floor apartment.
"It's still unbelievable to me," said the victim's sister Cindy Avarca.
Avarca's daughter actually suffered minor smoke inhalation in the fire. The mother says she believes her sister Roxana died trying to save others.
"That's the reason we think she passed away. She tried to take her kids; she wouldn't leave her daughter. She was left behind. She went back for her," Avarca said.
Firefighters were called to the three-story apartment building just after 1 a.m. Saturday. They rushed to save the families inside. Their task was complicated by the fact the fire started in the front stairwell and quickly destroyed access to the upstairs apartments.
Early signs indicated arson was the cause of the fire.
Firefighters went door-to-door Saturday afternoon in the city's Albany Park neighborhood handing out smoke detectors. It is believed the ones in the apartment building were working, but investigators say the intentionally set fire -- and the smoke it produced -- simply moved too fast for the young mother and her daughter.
"She was great, nice to everyone and loved her kids," Avarca told ABC7 Chicago.
Eight other people are now homeless as a result of the fire. The Chicago Department of Human Services reportedly is helping them find everything from clothing to a place to stay.