A four-story building being demolished in Center City Philadelphia suddenly collapsed Wednesday morning, killing three people and trapping at least 13, who were hospitalized after being removed from the debris.

The scene at 22nd and Market Streets is still active and dozens of rescue crews continue to search for possible survivors. A total of 13 people were hospitalized at nearby trauma centers with minor injuries and listed in stable condition.

Firefighters were conducting a "hands-only" operation, with no heavy equipment, for about the first hour. It was firefighters with buckets moving bricks. A team of four search dogs were also deployed at the scene.

MyFoxPhilly.com reported that it appears firefighters are split into two groups: one group focused on the back of the collapsed building, and the other group inside the nearby Salvation Army.

Marc Newell told the Philadelphia Inquirer that he heard an "unbelievable rumble," "like a freight train."

"You could hear the moaning," he told the paper.

The collapse involved a Salvation Army corner thrift store and a four-story building next door with a sandwich shop on the first floor. It's not clear whether any other adjacent buildings were damaged.

Carlton Williams of the city's Department of Licenses and Inspections said there were no existing violations on the building and the demolition company had proper permits for the work they were doing.

The city issued a demolition permit for the four-story structure on Feb. 1. Online records list the contractor as Plato Marinakos Jr., an architect. He told The Associated Press that Campbell Construction was handling the demolition. A message was left at a listing for Campbell Construction in Philadelphia.

Police said it appears the collapse was the result of an industrial accident.

Veronica Haynes was on the fifth floor of an apartment building across the street.

"I was standing there looking out my window, watching the men at work on the building, and the next thing I know I heard something go kaboom," she said. "Then you saw the whole side of the wall fall down ... onto the other building."

Bill Roam, a 54-year-old construction worker who has been working on a remodeling project at a nearby site for several days, heard the collapse and ran over. He said the dust was so thick that it was hard to see, adding that he was concerned in recent days by the way the building was being demolished.

High school student Jordan McLaughlan said he saw several people on the ground being given oxygen by rescuers after the collapse.

"It was hard to breathe, there was a lot of dust everywhere," McLaughlan said.

Read more from Fox News.