The railway company responsible for a runaway train that sparked deadly explosions in Lac-Megantic, Que. says a locomotive shutdown may have released the air brakes that were supposed to hold the 73-car train in place overnight.

Montreal, Maine & Atlantic Railway, or MMA, laid out the possible cause of the derailment in a statement Sunday afternoon.

“One fact that has emerged is the locomotive of the oil train parked at Nantes station was shut down subsequent to the departure of the engineer who had handled the train from Farnham, which may have resulted in the release of air brakes on the locomotive that was holding the train in place,” the company said.

MMA noted, however, that it does not have “complete information” about the chain of events that led to the disaster. The company said the government’s investigation into the derailment has “largely prevented” it from completing its own probe.

The Transportation Safety Board said Sunday its investigators have inspected the locomotive and recovered the train’s black box.

The TSB would not speculate on the possible cause of the disaster, which killed at least five people and destroyed half of Lac-Megantic’s downtown core.

What is known is that the train carrying crude oil was parked and locked down on the tracks in Nantes, about 13 kilometres from Lac-Megantic, late Friday night. The conductor left the train and went to a hotel for the night. 

Around 11:30 p.m., firefighters responded to reports of a blaze on the train’s locomotive. It’s not yet clear what caused that fire and what role, if any, it may have played in the subsequent derailment.

Donald Ross, the TSB investigator in charge, would only say that the initial fire is part of the agency’s “360-degree” investigation.

Shortly after 1 a.m. Saturday, the train somehow came unlatched and careened into Lac-Megantic. It came off the rails, caught on fire and set off massive explosions that pulverized buildings and sent residents running for their lives. 

Ross said the TSB will be looking at the state of the train’s air brakes and hand brakes. The train engineer will likely be interviewed by several different agencies looking into the crash, he said.

Five people are confirmed dead and about 40 others are unaccounted for. Police have warned that the death toll is expected to rise.

The disaster forced between 1,500 and 2,000 people from their homes.

Edward Burkhardt, president and CEO of Rail World Inc., the parent company of MMA, has said that he doesn’t understand how the train got away.

“If brakes aren’t properly applied on a train, it’s going to run away,” he told The Canadian Press. “But we think the brakes were properly applied on this train.”

MMA said its people “remain in a state of shock over this incident.” The company said it’s doing everything it can to assist affected residents and will co-operate with the government investigation.