Workers continue the cleanup in the Toronto neighbourhood near the propane plant that exploded on Aug. 10. (Mike Crawley/CBC)

It could take months to determine the cause of last weekend's propane explosions and fire at the Sunrise Propane plant in northwest Toronto, according to an investigator with Ontario's Office of the Fire Marshal.

The news comes as Ontario Premier Dalton McGuinty said the province is open to providing financial aid to residents whose homes were damaged.

"I said to [Deputy Mayor] Shelley Carroll, let's find a way to help the community, and if there is something that we might be able to assist in, we remain open to that," McGuinty said. "I didn't get into any specifics, but I just said that if we can help, you should call upon us and we'll take a look at it."

Almost all of the 12,000 residents forced to flee in the early morning hours have now been allowed to return to their homes.

Two more homes were released to their owners on Monday morning, after being inspected and having the surrounding property cleaned.

But the accident site is vast and investigators said they need to be mindful of dangers — including unexploded canisters of propane. Many tanks were damaged in the blast but remain half-filled with the gas.

Todd Clarke, an investigator with the fire marshal's office, told CBC News on Monday that to make matters worse the scene is heavily contaminated with asbestos.

"We're geared up for that, we're capable of working within that environment, so the guys that go into the scene are in [hazardous materials suits]. They have to go through hazmat [decontamination] going in and going out. It just makes the process a little longer, that's all," he said.

Clarke said his office will be on the scene for a few more days. After that, forensic scientists will take the evidence from the blast site for testing. Clarke warned against drawing any conclusions before all the evidence is in.

"After that, there's testing to be done with the evidence that they've seized. We'll be working in concert with the Centre for Forensic Science. They've been kind enough to be at the scene.

"They don't often have them right at the scene, but that testing, all these things, [it] could take months before we know," he said.

Living with regret

That's little comfort for people who live near the plant.

Maria Grossi, like others in the neighbourhood, has been dealing with shock, fear, grief and anger. Grossi regrets she didn't complain about the propane plant earlier.

"One thing that really makes me upset myself is that I felt I didn't do enough before," she said in an interview with CBC News. "I knew people were upset about it. I know people who wrote and complained and I didn't do enough."

Glenn Sears and his partner Cheryl said they've been told of other alleged problems at Sunrise.

"Our friend Danny went to have his tank filled and they overfilled it, put 45 in a 20 psi tank — and never mind the cigarettes — putting out the cigarettes in the booth saying, 'I'll be right with ya,' and butting out the cigarettes right on the site," said Sears.

Wyld Kostiuk, the general manager of Camcarb in north Toronto, said he doesn't allow any open flames at his operation, but he has an idea as to what Grossi may have seen.

"There is a technique used for draining cylinders for re-certifying them — because they have to be empty in order to be tested — that involves creating a burnstack to burn the propane off in the cylinder, excess that is left inside.

"We use a system to pump the propane out of the tank, it's safer, no spark no flame," he explained.

Kostiuk is also baffled about the explosions.

"My first opinion was there's foul play because the actual chances of a propane explosion like that are so slim. Propane is harder to ignite than gasoline."

Kostiuk said propane tankers are designed with emergency shut-off valves.

"[Propane tanks are] not something you just throw a match at and it lights. It has to be at a certain pressure, certain atmospheric conditions," said Kostiuk.

Alan Nuefeld, who also operates a propane-filling station and has been in the business for 30 years, said he thinks investigators will discover that "somebody did something they shouldn't be doing, especially at four o'clock in the morning."

Bob Leek, 55, Toronto Fire Services district chief of emergency planning, was found unresponsive at the scene of the blast and was pronounced dead at hospital. Authorities also discovered a body at the Toronto propane depot.