(CNN) — A vacant building being demolished collapsed onto a thrift store in Philadelphia Wednesday morning, trapping more than a dozen people under the rubble, some for hours, city officials said.
Two people remained trapped Wednesday afternoon under “tons of rubble,” Fire Commissioner Lloyd Ayers told reporters.
“We have located them and we’re going to continue until we can get them out and we can get them to hospitals,” he said.
Mayor Michael Nutter later said officials don’t know for certain if anyone else was trapped.
Twelve people rescued earlier were taken to area hospitals with minor injuries. Another woman walked away from the scene with no major injuries after being trapped for more than two hours, Nutter said, bringing the total of people freed from the rubble to 13.
University of Pennsylvania Hospital was treating four patients from the collapse, according to hospital spokesman Stephen Graff. Hahnemann University Hospital was treating two patients, according to hospital spokeswoman Gianna DeMedio. They were in fair condition.
The building collapsed onto the Salvation Army Thrift Store next door with an ominous rumble, witnesses said.
“You felt it shake,” Jordan McLaughlin told CNN affiliate KYW. “There was people that actually fell over. People started screaming, they ran across the street. There was people inside the building, you heard them scream.”
He said he helped two people out of the building. Other bystanders, including construction workers, helped four or five others out in the moments after the collapse.
Witness Ari Barker said he was in his office across the street when he heard “a rumbling, a very unusual sound.” He rushed to the window to see a plume of dust rising from the debris.
Another witness, Kate Slyman, said she felt the ground rumbling as the building collapsed.
“The first thing that came to my mind was a terrorist attack,” she said.
Philadelphia Police described the collapse as an “industrial accident. The Occupational Safety and Health Administration has been told it was an accident at a demolition site, and it has investigators on the way, spokeswoman Leni Fortson said
There were “no existing violations” at the building, a city building inspector said.
Rescue crews were digging through the debris “brick by brick,” said Mike Kenish, who owns a nearby tavern.
Nutter said the rescue effort would have to proceed delicately.
“This is dangerous work,” he said.
The collapse occurred Wednesday morning in a heavily traveled area of the Philadelphia’s Center City neighborhood. It happened near the Mutter Museum, a popular tourist destination that houses medical oddities.
The museum said on Facebook that it would be closed until further notice. While its building was undamaged, the museum said police were using its facilities as a staging area for the rescue operation.