(CNN) -- At least 13 people were killed and an estimated 37 others are still missing in the small town in Quebec where a runaway train exploded Saturday, Canadian police said at a news conference on Monday.
Investigators last reported 40 missing and five dead on Sunday. Police are counting as missing people whom direct family members have reported, Quebec provincial police spokesman Benoit Richard said.
"Hot zones" lingering more than two days after the train derailment in Quebec were hampering authorities' efforts to continue their search for missing people.
The unmanned train was carrying 72 tankers of crude oil when it plowed into the heart of Lac-Megantic, Quebec, on Saturday. The crash and a series of explosions flattened 40 buildings and forced nearly 2,000 people to evacuate the town, 130 miles east of Montreal.
More than 1,500 of those evacuated will be allowed to return to their homes Tuesday, officials said.
Among the buildings destroyed was the Musi-Cafe, which was hosting live music the night of the crash. Authorities have said they suspect some of the 40 missing residents were at the popular bar that night.
"There's been a lot of work that's been done on the scene during the night and hopefully we'll get some more areas we can search during the day, but that, of course, is under the firefighters' responsibility," Richard said.
Sunday, provincial police Lt. Michel Brunet said at least five burned bodies had been found, but "we know that there will be many more" deaths.
Canadian Prime Minister Stephen Harper described the town as a "war zone."
"There is not a family in this area that is not touched by this," Harper told reporters after touring the destruction Sunday.
Canadian Transport Minister Denis Lebel said officials are working to find out whether any safety violations contributed to the crash, and said he would take "immediate action" if so.
How did it happen?
The company responsible for the train said that after the crew parked the locomotive for the night at a station about seven miles from Lac-Megantic, the air brakes holding the train in place likely failed, allowing the train to barrel downhill into the town.
The Montreal, Maine & Atlantic Railway said the investigation is continuing.
"We don't know if the brakes were properly applied to the train, and we haven't been able to get into the fire area in order to inspect," Edward Burkhardt, president and CEO of Rail World Inc., the railway's parent company, told CTV.
Investigators with the Transportation Safety Board of Canada found the locomotive event recorder, which they can analyze for information on throttle position and speed, among other data.
The Quebec disaster came on the heels of a handful of other mishaps involving trains transporting oil in Canada, prompting some in the Canadian government to criticize what they say is a trend allowing rail lines to police themselves.
Among the incidents:
• Four Canadian Pacific rail cars carrying flammable petrochemicals used to dilute oil derailed on a flood-damaged bridge spanning Calgary's Bow River in June, according to the Calgary Herald.
• In another incident involving Canadian Pacific, five tankers containing oil derailed in rural Saskatchewan in May, spilling 575 barrels of crude, the Toronto Sun reported.
• The company also made headlines a month earlier when 22 rail cars derailed near White River, Ontario. Two of the cars leaked about 400 barrels -- almost 17,000 gallons -- of oil, The Globe and Mail in Toronto reported.
• Canadian Pacific was also involved in a stateside spill in March. Fourteen cars on a milelong, 94-car train derailed in western Minnesota, about 150 miles northwest of Minneapolis, spilling about 30,000 gallons of crude, Reuters reported.
"We're seeing more and more petroleum products being transported by rail. There are attendant dangers involved in that," Thomas Mulcair, leader of the opposition New Democratic Party, told CTV.
Mulcair criticized the government for "cutting transport safety in recent years."
Emile Therien, a former president of the nonprofit Canada Safety Council, added, "In the last 10 years, the railroads really do their own safety thing. There's very little involvement from Transport Canada. Transport Canada's got to get back into the game; there's no doubt about it."
Lebel said railway companies are required by law to ensure the safe operation of their trains. He also said that since 2007, the number of train accidents in Canada has decreased by more than 22%.
The train involved in the Quebec incident was carrying oil from the Bakken oil formation in North Dakota to a refinery in New Brunswick, Canada. According to the Association of American Railroads, Bakken is partially responsible for a dramatic increase in the amount of oil moved via train since 2008.
As U.S. crude production has increased with advances in horizontal drilling and hydraulic fracturing, or fracking, "much of the recent increases in crude oil output has moved by rail from production areas, such as North Dakota, that are not adequately served by pipelines," according to the AAR, whose members include the major North American freight railroads.
In 2008, the group said, U.S. railroads were carrying about 9,500 rail carloads of crude oil. By 2011, it was 66,000. Last year, it was more than 200,000 carloads. A railcar carries about 700 barrels of oil, with 42 gallons per barrel.
Balls of shooting flames
Witnesses told CBC they heard five or six explosions Saturday in Lac-Megantic. One person saw the train's first oil tanker tip over and yelled "run, run!" as he dashed toward a lake. The flames chased him to the edge of the water.
"The fire was moving so quickly," he said. "We saw balls of fire shooting out onto the water."
One woman told CTV that she got off work at a nearby bar an hour before the accident.
"I have no news from my friends; I haven't heard from any of them," she tearfully told CTV. "I can't say more than that. We're waiting for confirmation."
Families and friends are scrambling to find the missing.
More than 17,000 people have joined a Facebook page to help people connect with their loved ones in the town.
Multiple posts ask about Guy Bolduc, a singer who was performing at Musi-Cafe.
"All of his fans, all over Quebec, but also his fellow singers (of whom I am one) hope to see him again alive!!! Come on my GuyBol, come out of your hiding place," one member wrote.
Residents struggled to absorb what had happened to their small lakeside community.
"It's dreadful," Claude Bedard told CBC. "It's terrible. The Metro store, Dollarama, everything that was there is gone."
Authorities evacuated more than a third of the town of 6,000 people, most from the center of Lac-Megantic and a home for the elderly.
Amanda Gabrielle said the train crashed on her birthday. She lost her dog and her home, and doesn't have any family or friends nearby.
"I lost everything," Gabrielle told the CBC. "I don't know what's going to happen to me."
CNN's Umaro Djau, Jonathan Mann, Pierre Meilhan and Deanna Hackney contributed to this report.