Updated

Officials say insufficient brake force was applied before an oil train slammed into a town in Quebec on July 6 and killed 50 people.

Donald Ross, chief investigator for the Transportation Safety Board of Canada, said Friday the insufficient brake force could be due to mechanical problems with the handbrakes, or a problem with the way someone applied them.

The unmanned train belonging to the Montreal, Maine & Atlantic railway carrying 72 cars of crude oil slammed into the heart of Lac-Megantic, setting off massive explosions.

A spokesman for the agency says it's had a closer look at 25 tanker cars since gaining access to the blast site two days ago — and has taken pictures and samples.