According to State Fire Marshal Joe Thomas, eyewitnesses told investigators that the fire started at the front porch of the apartment near University of Southern Maine yesterday.
The five deceased were yet to be identified, officials said, adding that none of the victims were university students.
The University of Southern Maine said there were students in the building, but there were no information of injury to any of them.
Seven tenants escaped the fire by jumping off from the building.
More From This Section
One man jumped from the second floor to escape the inferno, but suffered severe burns, a local news channel said.
The cause of the fire is yet to be ascertained.
"This investigation will last until it ends and that could be several days. With the amount of agencies involved, the extent of the damage and the number of victims, there are a lot of parts to this investigation," Portland Fire Chief Jerry LaMoria told reporters at a news conference.