A man was working on the top floor of this this tower, under construction, when he was crushed by a load of building materials. (Bryan Labby/CBC)

A worker on a downtown Calgary construction site died when he was crushed by 1,800 kilograms of building materials that slipped out of a crane sling.

The man was working as a rigger, which assists the tower-crane operator with moving material, on the 21st floor of the Penn West tower, at 215 9th Ave. S.W., on Friday afternoon, said duty Insp. Frank Reuser of Calgary police.

A sling was being used to move heavy concrete forms from one part of the top floor to another when the load shifted and fell out, crushing the 27-year-old man, said Reuser, correcting an earlier police report that the worker fell 10 storeys.

The man was pronounced dead at the scene.

"The whole building shook, and as far as I know, a form fell on a man," said one worker leaving the tower.

The worker was crushed when building material shifted and fell on him. (Bryan Labby/CBC)

Ryan McConnell said he and a colleague were just wrapping up placing concrete at a site across the street when they heard a loud bang from the Penn West tower at about 2 p.m.

"All of a sudden, the wind picks up, the rain picked up. You hear a thud, and their rigger on the ground here just started screaming 'I need a medic! First aid, first aid!" then all hell broke loose," McConnell told CBC News.

Occupational Health and Safety investigators and the chief medical examiner have now taken over the investigation, said police.

Construction workers said they've been battling the weather in the past few days.

"It's been windy, it's been rainy, it's been nice, it's been snowy, and it makes for really rough work conditions," said McConnell. "You've got to be safe, be aware of your surroundings, know what's going on, because at any time something bad could happen. You just don't know."

"Some days, it's easy; some days it's not," said his colleague Paul Abreu.

With files from Bryan Labby