Six people died and 14 were injured after a four-storey building under demolition collapsed in downtown Philadelphia, according to the officials.
The collapse happened Wednesday at around 10:30 local time where the building under demolition suddenly fell down over a thrift store adjacent to the building.
Amid search and rescue efforts, Mayor Michael Nutter told reporters that the number of casualties could rise as it was still unknown how many people were inside the thrift store or on the side walk when the building suddenly came down, burying the store and the pavement.
"We still do not know how many people were inside the thrift store or possibly on the sidewalk at the time of the collapse," said Michael Nutter while speaking at a press conference following the building collapse.
"If someone else is in that building, they (rescue teams) will find them," said Nutter.
Witnesses at the collapse earlier have told reporters that they had heard a loud rumbling noise seconds before the collapse.
"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," said Veronica Haynes, who was in a nearby apartment building at the time of the collapse.
"Then you saw the whole side of the wall fall down... on to the other building,'' said Veronica Haynes.
Some witnesses even said that they had been complaining about the way that the demolition was being carried out before the incident happened.
"We've been calling it for the past week - it's going to fall, it's going to fall,'' said window washer Dan Gillis, according to the Associated Press.
Authorities said that they were still investigating the cause of the collapse that occurred at 22nd and Market streets in the heart of Philadelphia's busy Center City district.
