A man in his fifties was killed when a retaining wall collapsed in Montreal's Rosemont–La Petite-Patrie borough.

The incident happened at a building that was under renovation on Marquette Street at about 11:20 a.m.  ET on Thursday. 

Firefighters said the man had entered the building to fetch some of his tools.

"He was alone when he went to get some tools, and the entire north wall fell on him," said Montreal Fire Department chief Mario Drolet. 

'We saw the wall — all the brick went down,' - Marc Bourg, eyewitness

Eyewitness Marc Bourg said the collapse was sudden.

"We saw the wall — all the brick went down. There's no chance [he survived]," Bourg said, who happened to be walking by the building at the time of the incident.

"They're doing construction there. There is one block, and another block, and in the middle, they are doing excavation, and the wall fell down on a guy."

One person on the scene was treated for nervous shock.

First responders said it's not clear why the wall suddenly collapsed, and investigators are trying to determine if the wall might have been built improperly.