The Ontario Fire Marshal's Office has ruled out a natural gas explosion as the cause of a fire at a rooming house in downtown Sudbury last Wednesday that left two people dead and 15 others homeless.

James Allen with the Ontario Fire Marshal's Office said investigators are still trying to figure out the cause of the blast.

"It was a substantial explosion," he said.

"We do have witnesses that say the building actually shook."

Allen noted the explosion happened inside — and sprayed a large field of debris.

With natural gas as a possible cause now ruled out, Allen said the explosion could also have been caused by propane tanks or other flammable fluids, but he wouldn’t speculate further.

Sudbury Fire Chief Dan Stack said the age and condition of the rooming house made fighting the fire difficult.

"Well it's heavy timber construction … it's built in 1915," he said.

"With sawdust and newspaper as the insulation, it was tough to fight the fire."

The Fire Marshal's Office confirms it has sifted through all remaining sections of the building and has found no more bodies.

Meanwhile, the identities of the two fire victims have not been released. Post-mortem exams are expected to be done at Health Sciences North in Sudbury on Tuesday.

Greater Sudbury Police report they are continuing with their investigation.