By Greg Argos

PHILADELPHIA (CBS) —  It has been one week and one day since a four-alarm fire tore through a block in Old City but the Philadelphia Fire Marshal’s Office and agents with the Bureau of Alcohol Tobacco Firearms and Explosives still have not been able to get inside the property.

“We’re a little delayed on the actual scene work because of the safety issues involved. As you know it’s a five-story building and unfortunately, during the fire, all five floors collapsed,” said Donald Robinson, the ATF special agent in charge.

In an exclusive interview with CBS3, Robinson says his department is looking into all possibilities of what may have started the fire, including arson.

City: Fire Left Philadelphia Building ‘Imminently Dangerous’

“Nothing has been ruled out. It’s wide open,” said Robinson.

But the priority now is stabilizing the adjacent buildings and ensuring investigators are safe when they are finally able to enter– hopefully by the end of this week.

“I’m not going to lie to you, it’s a lot harder. If those floors had stayed intact, we would be able to get in there. It would be a lot easier to do that. We’ve got to painstakingly look at each level. Remove each level and floor and then do another examination and then work out a way to the bottom,” said Robinson.

Right now agents are conducting interviews and reviewing surveillance video.

“I would say there are a lot of people we’ve talked to. We’ve received a lot of information on tips. And through those interviews, and we’re checking them out,” Robinson said.

But the key to finding out what happened, will happen, when agents step foot inside.

“All the interviews and everything is very important, but until we get into the scene we can’t put the whole picture together,” said Robinson.