A building has collapsed in an apparent accident at a demolition site, at 22nd and Market streets in Philadelphia, Pennsylvania, Wednesday, June 5. 
An injured woman is taken to an ambulance after the building collapse. 
Firefighters search through the rubble looking for survivors. 
iReporter Mike Adam shot this photo of the collapse from his apartment building across the street. 
This image from Google Maps shows the building, seen on the left, before the collapse. - NEW: At least two people remained trapped, a fire official says
- NEW: Twelve people are taken to hospitals with minor injuries, officials say
- NEW: The rescue effort is "delicate" and "dangerous," the mayor says
- NEW: The building was being demolished when it collapsed, authorities say
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(CNN) -- A vacant building being demolished collapsed onto a thrift store in Philadelphia Wednesday morning, briefly trapping more than a dozen people under the rubble, city officials said.
Two people remained trapped under "tons of rubble," early Wednesday afternoon, 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.
The 12 others rescued earlier were taken to area hospitals with minor injuries.
Collapse signaled by 'unusual rumbling' 
Witness describes how building fell 
Victims trapped in building collapse "It is delicate. This is dangerous work," Mayor Michael Nutter said.
The building collapsed onto the Salvation Army Thrift Store next door with an ominous rumble, witnesses said.
"I was in my office in the high rise across the street and felt and heard a rumbling, a very unusual sound," witness Ari Barker said.
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."
There were "no existing violations" at the building that collapsed in Philadelphia on Wednesday, injuring at 12 people, a city building inspector said.
The collapse occurred Wednesday morning in a heavily traveled area of downtown Philadelphia near the Mutter Museum, a popular tourist destination that houses medical oddities.
The museum was closed Wednesday due to the collapse, it said on Twitter.
As rescue efforts continued, 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.
Bystanders and construction workers rushed in to search for people trapped under the rubble, Barker said.
Rescue crews soon arrived and asked the public and news helicopters to back off to allow searchers to listen for people trapped under the debris.
"We're asking the public to stay out of the area to allow the rescue workers access," the police department said in a tweet.
The Occupational Safety and Health Administration has been told it was an accident at a demolition site, and it has investigators on the way, representative Leni Fortson said.
A witness, Jordan McLaughlin, told CNN affiliate KYW, that a building "collapsed the wrong way and landed on a thrift shop" that had people inside.
McLaughlin said he and others rushed to help people out of the rubble. He said he helped two people out, and saw up to five others escape within the first 10 minutes.
The fire department was called to the site at 10:43 a.m., Capt. Jeffrey Thompson told CNN. Thompson said it wasn't immediately clear whether it was an apartment building or an adjacent building scheduled for demolition.
Video from WPVI showed two people helping a third person, apparently hurt, away from the rubble. It also showed emergency personnel walking on and around rubble on an exposed area of a ground floor
