That determination came from the Bristol Township fire marshal Kevin Dippolito on Monday, who was investigating the fire at the Magic Cottage Day Care.
The case has been turned over to police. There has been no word on any suspects or a motive for setting the fire.
The fire was reported around 2:00 p.m. at the facility in the 100 block of Appletree Drive in the Levittown section of Bristol Township.
The fire burned for about 40 minutes before it could be brought under control.
Investigators earlier said they believe the fire started in a classroom being used for storage.
Two firefighters suffered non-life-threatening injuries.
The students who were enrolled in the day care were sent to the center's Yardley location on Monday while the cleanup continued.