MEGANTIC, Quebec, Sha'ban 29, 1434, Jul 8, 2013, SPA -- Hazardous conditions slowed firefighters' attempts Monday afternoon to search for some 40 people still missing after a runaway oil tanker train exploded over the weekend in a Quebec town, killing at least five people and incinerating at least 30 buildings, AP quoted officials as saying. Richard Gagne, a spokesman for Quebec provincial police, said it was still too risky to begin searching the devastated downtown for the missing while firefighters made sure all flames were out and unexploded oil tankers were kept cool. The crash raised questions about the safety of Canada's growing transportation of oil by train. All but one of the train's 73 tanker cars were carrying oil when they somehow came loose early Saturday, sped downhill nearly seven miles (11 kilometers) into the town of Lac-Megantic, near the Maine border, and derailed, with at least five of the cars exploding. It was not clear how fast the cars were moving when they derailed. Queen Elizabeth II expressed deep sadness over the disaster Monday, saying in a message through the federal government that the loss of life "has shocked us all." Canadian Prime Minister Stephen Harper toured the town Sunday and compared it to a war zone. The train's owners said they believed brake failure was to blame. "Somehow those brakes were released, and that's what is going to be investigated," Joe McGonigle, Montreal, Maine & Atlantic Railway's vice president of marketing, said Sunday. Officials were also looking at a locomotive blaze on the same train a few hours before the derailment.