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Joseph Kaczmarek, Associated Press
Firefighters battle a fire in a warehouse on York Street near Kensington Avenue, early Monday, April 9, 2012, in Philadelphia. Officials say the fire claimed the lives of two Philadelphia firefighters after a wall collapsed inside an adjacent building where the two men were trying to halt the fire's progress.

PHILADELPHIA — Philadelphia fire marshals are sifting through the rubble in search of what caused a fire at a vacant warehouse that left two firefighters dead.

City officials say the warehouse had been cited for fire code violations several times. 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.

Executive Chief Richard Davison said Tuesday it could take a long time to determine the cause, especially considering the size of the massive six-story building. He says there's no way to know how long it will take.

City officials say they expect to meet Tuesday with attorneys for the building's owners. An attorney for the developers has called the fire "an unspeakable tragedy."