The roof caved in around 2:20 p.m. Saturday at the Algo Centre Mall in Elliot Lake, which is about 150 kilometres west of Sudbury.
In an interview with CBC News, Elliot Lake Mayor RIck Hamilton said four people were rushed to hospital, though none of the injuries were believed to be life-threatening.
He was not able to confirm whether there were any fatalities.
A special Ontario Provincial Police "urban extraction" unit was dispatched to the site in case anyone was trapped and needed to be rescued. The unit was expected to arrive Saturday night, and officials said they would probably not have word of any possible deaths until Sunday.
The part of the roof that caved in served as a parking area for customers and mall staff.
The roof's crumbling was inadvertently caught on tape by the editor of the Elliot Lake newspaper, the Standard. The editor was at the mall at the time covering an event. In the audio recording, the mall's fire alarms go off and people begin evacuating the scene after what sounds like the crash of a massive wave.
"Don't — don't go that way," someone says on the recording. "Check that door, in there, is an exit, please. Check it, please," the same voice says.
Joe Drazil, a Zellers employee at the mall, said cars appeared to have fallen through a gaping hole near the escalators.
"You can see the roof with the cars hanging inside," he said.
Gas leak prompts power shutdown
Emergency officials turned off the mall's power due to a gas leak caused by the collapse.
Rhonda Bear, the Algo Centre Mall manager, told CBC News that the mall has been doing repair and maintenance work on parts of its roof over the last year, but "there hasn't been any huge structural repairs" to the section of the mall that gave way.
She also said that the owners of the mall, Toronto-based Eastwood Mall Inc., ordered an engineering and structural study on the nearly 30-year-old building, parking lot and adjoining hotel a month ago.
"It turned up nothing," Bear said.
In April, the Elliot Lake Standard newspaper published a letter by Bear saying that "the owners have been working very hard to bring the mall to good standing and rebuild the mall's reputation to a positive one."
Bear wrote in her letter that owners have poured $120,000 into mall repairs as part of a "facelift" project over the prior 12 months.
Asked on Saturday how crowded the mall was when the cave-in happened, Bear said it was less busy than usual for a Saturday.
'Plagued' with leaks
The Standard newspaper has previously reported that the mall had been "plagued" with leaks from the roof for years.
About four years ago, the mall's owners embarked on a $1-million overhaul of the roof to end the leaks, which were forcing merchants to shut their stores as well as damaging their wares. An architect on the project warned that if moisture accumulated in some of the fibreboard pads lining the roof's concrete slabs, the pads could deteriorate.
The newspaper also reported that part of the ceiling in a mall restaurant collapsed due to the leaks, and that the roof's patchwork of repairs was "easy to see."
In March, the mall's managers pleaded guilty in provincial court to having sprinklers and fire alarms that failed to meet the fire code. Management spent $50,000 to upgrade the fire-prevention infrastructure to avoid potentially heftier fines.
Among the businesses and offices at the mall is a Dollarama outlet, the Elliot Lake Public Library and an office for the federal Department of Public Works. An 80-room hotel also operates at the mall.
A 10-member OPP extraction unit from Bolton, Ont., was on site.
"We need to meet with them and the Ministry of Labour to conduct site assessments," Hamilton told local radio station CKRN FM.
Info centres to report missing
"It's important to secure the site to be able to look for any people who may be missing," he said, adding that rescue operations would be underway as soon as the site has been confirmed as secure.
Hamilton called a state of emergency, reasoning that the declaration allowed him to call in provincial rescue units and have full authority to secure the site.
In the meantime, police are asking anyone who believes they may know someone who was at the mall and unaccounted for to report them at an information centre.
"We have OPP officers there along with vicars and critical incident stress people that are there to assist them. And we want to know if there is anyone missing," OPP Const. Marilyn Cameron said.
People who fear someone may be missing can call 1-888-310-1122.