Canadian authorities said the death toll from last weekend's train derailment and explosion in the eastern town of Lac-Megantic has risen to 28.

Quebec Provincial Police inspector Michel Forget said Friday in a press conference that rescue teams had recovered four more bodies in recent hours in one of the areas of the town most affected by the accident.

Forget said the number of fatalities could rise to 50 because another 22 people are still missing.

He also said the process of recovering human remains and the investigation into the accident could be prolonged because of difficulties the teams are experiencing.

On Friday, the search for bodies slowed to a crawl due to air pollution in the wake of the accident, in which a runaway train carrying 7,200 tons of crude oil rolled downhill, derailed and exploded near downtown Lac-Megantic in the wee hours of Saturday, July 6.

Several tank cars exploded in flames, destroying some 30 buildings in Lac-Megantic, a town of 6,000 inhabitants located 250 kilometers (155 miles) east of Montreal.

The spilled oil burned for several days until firefighters managed to extinguish the remaining flames.

Edward Burkhardt, chairman of the Montreal, Main and Atlantic Railway, the company that operated the driverless freight train, has blamed off-duty engineer Tom Harding for the accident, saying he did not apply all handbrakes necessary to keep the train parked before leaving it unattended and going to sleep at a hotel in Lac-Megantic.

But the director of the Transportation Safety Board of Canada, Wendy Tavos, said the cause of the accident was surely more complex.

"No accident is ever caused by one thing," she said. "It is always a series of things and always involves the organization and the way that they operate. It never comes down to one individual."