Cause of massive 4-story vacant West Philly apartment building announced

WEST PHILADELPHIA - After an investigation, officials with the Philadelphia Fire Department have announced a cause of the four-alarm, vacant, four-story apartment building that produced massive flames and smoke Saturday.
What we know:
The Philadelphia Fire Marshal’s Office found that the four-alarm fire in a vacant four-story apartment building that erupted in flames and heavy smoke Saturday morning was deliberately set.
Officials said the fire was incendiary and the case has been referred to the Philadelphia Police Department.
The backstory:
Fire crews got the call for a fire burning in a four-story apartment building on the 200 block of South 48th Street early Saturday morning, just after 5 a.m.
Once on site, firefighters quickly escalated the alarm from 2nd to fourth, as smoke and flames just poured from the very large, vacant building.
As two hours passed, as crews were tamping down hot spots, new flare-ups occurred, complicating the efforts in bringing it under control.
Firefighters did eventually bring it under control, but remained on scene until late into Sunday dousing hot spots.
What we don't know:
Philadelphia Police officials have not released details regarding their investigation into the arson.