The fire broke out shortly before 1:30 and was brought under control in about 23 minutes, Fire Commissioner Lloyd Ayers said.
Ayers said the fire marshal was waiting to talk to Tasha and Tyheed Rose before making a determination on the cause of the blaze.
Firefighters were called to the 19-story public-housing building by the complex's managers, who were notified by the building's alarm system. Heavy smoke was pouring out of the sixth floor, Ayers said.
Tasha Rose, the neighbor, and a firefighter were taken to the Hospital of the University of Pennsylvania with injuries sustained in the rescue. The neighbor and firefighter were later released from the hospital. Tyheed Rose also was taken to Children's, where he was in stable condition. Ayers did not know the condition of the mother but said she was not in stable condition.
Neighbors at the West Park complex said a third son was at school at the time of the fire and came home to the news of the fire and his brother's death.
Twenty have died in fires in Philadelphia this year, Ayers noted, compared with 28 at this time last year. He said that about 85 percent of fire-related deaths involve housing with no smoke detectors or other response systems.
"This is different," Ayers said, because alarm systems went off and people were evacuating. "Everything worked here," he said of the response system.
About 300 residents live in 109 units in the building, according to Philadelphia Housing Authority spokeswoman Nichole Tillman. There are three buildings in the complex, with more than 800 residents in about 300 units.
All residents were allowed back into the building by Tuesday evening, according to a PHA official. The only unit not reopened was Apartment 603, where the fire occurred. The family will be provided another unit, the PHA official said.
Some nearby units on the fifth and sixth floors sustained water damage, the official said, but all were reopened Tuesday.
PHA's interim executive director, Kelvin Jeremiah, said in a statement early Tuesday evening that "the surviving family members have experienced an unspeakable loss, and our entire community must support them during this very difficult time."
Jeremiah also praised the Fire Department's quick response and thanked "city response workers," including PHA staff, the American Red Cross, and the Salvation Army.
Red Cross director of recovery Chad Lassiter said the organization was on the scene soon after the fire began, offering auxiliary services. The Red Cross also set up a "comfort center" in a community center in one of the other buildings, Lassiter said, where around 15 residents were helped by staff members, who offered food, water, and blankets.
"What's most essential right now is mental health services," Lassiter said. "Your heart always bleeds when it's children."
Contact Jonathan Lai at 215-854-2771, firstname.lastname@example.org, or on Twitter @elaijuh.
Inquirer staff writer Robert Moran contributed to this article.