A Canadian mining company that employed workers who were fatally ambushed Wednesday in the West African nation of Burkina Faso offered no explanation on Thursday for the attack or the possible motives of the killers.

But in a securities filing made this week in Canada, the Montreal-based company, Semafo, characterized two attacks on its workers in Burkina Faso last year “as part of an increase in similar incidents carried out by bandits or terrorists.”

The company added that “there is no reason to believe that our employees or operations are targeted.”

At least thirty-seven people died in the Wednesday attack, one of the deadliest episodes of violence that has convulsed the landlocked nation. It took place when five buses escorted by Burkina Faso’s military were traveling to Semafo’s Boungou mine, an open-pit gold mine in the eastern part of the country, about 25 miles away.