Arson likely triggered the massive fire that ravaged northern Alberta's Slave Lake last spring, a provincial investigation has found.

While officials haven't said if the arson was deliberate or the result of recklessness, the file has now been passed onto the RCMP.

"Our investigation eliminated all natural, industrial or accidental causes," said Sustainable Resource Development Minister Frank Oberle Tuesday.

"It's extremely disappointing and my heart goes out to the people that lost homes or property," he added. "This further exacerbates that."

Alberta's investigation took five months. The fire swept into Slave Lake on May 15, engulfing about one-third of the community and leaving around 2,000 people homeless.

"Our investigation took the time required to protect, collect, analyze and document evidence to the rigorous standards required for preparation in a court of law," Oberle said.

The Slave Lake wildfires caused an estimated $700 million in damage, making it the second most-costly insured disaster in Canadian history after the 1998 Quebec ice storm, which cost nearly $2 billion.

The fire forced all 7,000 residents out of the town, which is about 250 kilometres northwest of Edmonton, for two weeks.

No one was seriously injured in the fire, but a helicopter pilot involved in fighting the fire died in a crash.

However, while officials were firm in their certainty that the blaze was not natural, many details remain unknown.

"We know there was no lightning at that site, no power line malfunction and no campfire located at that site, so a process of elimination gets us to the only reasonable conclusion," Oberle said.

Not only did the minister decline to say where the fire originated, but he also refused to describe the natural surroundings of the area pinpointed by investigators. He would not say if the area was in the forest or near a campsite.

Alberta has already delivered about $300 million in aid, much of it used for modular homes, but locals said that whoever is responsible should be punished.

"The most you can do would be put him in jail," resident Dwayne Verschoor told The Canadian Press. "I would string him up on Main Street -- but we're supposed to be civilized people."

Still, some residents said that despite the latest developments, nothing can change what happened.

"It's going to make no difference who they catch, blame, whatever," nursery school teacher Karen Scharf told CP. "It's going to make no difference in my life, because my house is gone."

According to RCMP Supt. Marlin Degrand, who is the chief of Alberta's criminal investigations, it will be "difficult" to build a case and collect evidence so many months after the fire.

"After a review we will determine an appropriate course of action," he said.

Meanwhile, the town is still trying to recover after parts of it were completely wiped out. Some homes have been rebuilt, but there are still many locals who will remain in mobile homes through the long winter.

As the police launch a criminal probe into the fire, a University of Alberta law professor explained that arson charges can stem from both deliberate and negligent actions.

"Arson is an ambiguous term," professor Steven Penney told CP.

The fire attracted international attention, and in July, Prince William and his wife, Kate, visited Slave Lake to show their support during their cross-country tour.