There will be no quick answers.
Investigators are focusing on a couple of spots on the York Street side of the building.
A crane has pulled out piles of debris and the investigators were going through it with shovels, looking for clues.
So far there have been none of the telltale indicators of arson.
"If there were multiple points of origin that would indicate a set fire, we don't have any of that in this case yet," ATF Special Agent Donald Robinson said.
But nothing has been ruled out.
The blaze broke out early Monday and two firefighters - 60-year-old Lt. Robert Neary and 25-year-old Daniel Sweeney - were killed in a building collapse at a nearby furniture store where the flames had spread.
The investigation is led by the Arson Task Force which involves federal ATF agents, fire marshals, and the Philadelphia Police Department.
They've interviewed witnesses and first responders to help isolate the possible point of origin.
As their work continues, the neighborhood also works to restore itself.
Several homes were burned out.
One family left with rescued belongings in shopping carts.
"Nothing but embers inside. I believe it started from the back of the house, so it worked its way forward," landlord Bill Avera said.
Volunteers went door to door advising homeowners to check their roofs for holes where embers might have burned through.
Back at the fire scene, investigators continue their relentless search.
"I can tell you everybody's working very hard. We lost two really important people and we want to get the answers," Robinson said.
It is painstaking work and it may take many more days before they get close to an answer.