UPDATE: Cause in fire determined
The fire was reported just after 2:00 p.m. at 545 Herman Street.
"We heard the screaming, it's a terrible sound to hear, and wish we could had done more to get them out of there," said David Haye, who tried to rescue the children. He and other Good Samaritans told the teens - a boy and a girl - to jump onto a backyard tree from the second floor and climb down.
However, they couldn't get to the four-year-old boy who later died.
"There was a futon stuck in the window," said Michael Vines. "So, I'm trying to pull the futon out of the way so the boy could get out but there was so much smoke."
Firefighters found the boy in a second floor room badly burned and unconscious.
The fire was out in about 20 minutes. Now, the investigation is under way.
"If you could see the burn, the level of burn, the amount of char, all of those things, you just wonder how they did it," said Fire Commissioner Lloyd Ayers.
The fire marshal worked into the night to determine the cause of the fire. Earlier in the day authorities said federal investigators would help in this case but, by Saturday night, they were not yet seen at the site of the fire.
"We have some questions, we're not sure what the cause is right now but we are going to find out what happened," said Ayers.
The fire department says there were no working smoke alarms in this home.
The American Red Cross said it was assisting three people as a result of this fire.
To date, 26 people have been killed in fires in Philadelphia compared to 17 this time last year.