Numerous residents have been displaced.
At least 20 hurt in West Chester fire
Fire officials have refused to say if there were any fatalities, and District Attorney Tom Hogan said it could likely be Saturday before crews are able to get inside.
Investigators from the ATF will be involved due to the size of the fire.
RAW VIDEO: Daytime aerials of West Chester fire scene
The blaze was reported at 10:46 p.m. Thursday at the Barclay Friends Senior Living Community on the 700 block of North Franklin Street.
RAW VIDEO: Chopper 6 video of West Chester fire
The fire quickly spread to multiple buildings and flames could be seen shooting from the rooves and windows of the structures.
Residents were forced to evacuate outside into the cold, with temperatures overnight hovering in the low 40s.
RAW VIDEO: Fire at senior community
Officials said the inferno went to five alarms before it was declared under control at 1:22 a.m. Friday.
Video showed dozens of residents on the lawn or along the street, wrapped in blankets. Many of them had been pushed in wheelchairs or rolled on beds to safety.
Fire at West Chester senior community
At a news conference shortly before 6:30 a.m. Friday, officials declined to say how many residents were injured, or if there were any fatalities.
But earlier in the morning, Chester County emergency officials said at least 20 people were taken to area hospitals for treatment.
RAW VIDEO: Ambulances arrive at Paoli Hospital
Action News is told nine people were taken to Chester County Hospital. Seven were taken to Paoli Memorial Hospital.
There was no immediate word on the conditions of the injured.
A spokeswoman for Main Line Health said one of its hospitals also was prepared to provide shelter to some additional residents of the senior living community.
According to its website, Barclay Friends offers various levels of care including memory care, skilled nursing, and post-acute rehab.
Fire at West Chester assisted living community
Early Friday morning, the local chapter of the American Red Cross said on Twitter that it had multiple teams on the scene and that it was working with county officials and facility staff to assist displaced residents.
A cause of the fire remained under investigation.
The Associated Press contributed to this report.