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The Elliot Lake inquiry has heard more about how challenging it was to assess the situation inside the collapsed Algo Centre Mall when rescue workers arrived at the scene.
The inquiry is now hearing from members of a heavy urban search and rescue team based in Toronto. TF3 specializes in getting people out of collapsed structures.
Team co-ordinator Tony Comella told the inquiry there were a lot of unknowns about the mall when the team arrived.
"There wasn't something that hit it or there wasn't an explosion, by all accounts," he said.
"So for me, there was a big question mark as to why it really happened in the first place and, of course, that leads you to wonder, how safe is the rest of the building?"
The inquiry is expected to hear more Thursday about what was happening behind the scenes when the rescue effort was called off about 48 hours into the search.
The inquiry has heard engineers were concerned the building had shifted and a beam in the area of the collapse was dangerously overstressed.
Comella told the inquiry TF3 works with engineers during rescues, but those engineers play an advisory role.
"The point of an engineer is not to say, 'a building is safe, you guys can go in there'... how would he possibly do that?" Comella said.
"It is a collapsed structure. Nothing is the way it is supposed to be. The intention is for him to assist us in understanding that building and offer his expertise."
Other witnesses have told the inquiry TF3 was set to leave Elliot Lake until public and political pressure prompted the rescue effort to resume.
In the coming weeks, the inquiry will hear from engineers who assessed the collapsed mall during the rescue.
The public inquiry was established in July 2012 by the Ontario government and has been underway in Elliot Lake since March.
It was created to report on events surrounding the mall roof's collapse on June 23, 2012, the deaths of Lucie Aylwin and Doloris Perizzolo, the injuries to others and the emergency management and response.
The hearings in Elliot Lake are scheduled to wrap up in early October. Former Premier Dalton McGuinty is scheduled to be the final witness.