Some experts say shoddy demolition practices by Campbell Construction may be to blame for yesterday's building collapse in Philadelphia that killed at least six people.

One building that was being torn down collapsed onto a second one, which housed a Salvation Army Thrift Store.

Paul Dengler of Dengler Demolition has torn down buildings like the one at 22nd and Market Streets before.

He says the key is propping up walls and floors with massive wooden beams, known as "shoring."

"For me looking in the pictures, because you can see inside the building, you can see where they used no additional shoring," he said. "They were just working with what's there, trying to be real careful and take it down a little bit at a time and it must've went on them."

Dengler says witness accounts and photos make the work look, in a word, "unprofessional."

"As close as it is to an existing building that's occupied, it has to be taken down by hand and they said there were big machines removing rafters," he said.

Drexel University associate engineering professor Robert Brehm says demolition can be unpredictable. But with officials scouring the remains of the neighboring Salvation Army thrift store, Brehm wonders what kind of precautions were taken.

"When you're taking down an older structure like that, that's adjacent to other structures, I would hope that there were procedures in place to secure those adjacent buildings, which would include evacuation of personnel," he said.

For now a spokesman says the Salvation Army is not making a statement.

Last night, Mayor Michael Nutter seemed to disagree with Brehm's view, telling a news conference that it's commonplace in Philadelphia, a city with many decaying structures, to have active demolitions adjacent to occupied buildings.

"We don't know why that particular wall came down, but I think if you talk to anyone in the construction or demolition industry, buildings are taken down or even built right next to active buildings or even residential structures," he said.

Brehm and Dengler each stressed how dangerous demolition work can be. Drexel's Brem described the business of tearing down buildings this way:

"It's not a science."

WHYY/NewsWorks Susan Phillips contributed reporting to this story.

Raw video below from YouTube user shows the demolition of Hoagie City three days before the building collapse.