Several ambulances and police cars are at the Loblaws, or Western Grocers, warehouse at 16104 121A Ave.
The attack happened at about 2:15 p.m. MT.
Shelly Paulson, who works across the street from the warehouse, said they were told by police to lock the doors and stay inside.
Police identified the suspect as Jayme Pasieka, an employee at the warehouse, and said he is considered extremely dangerous and is the subject of a massive manhunt.
One witness reported seeing the suspect, dressed in military garb, fleeing the scene.
Loblaw would not offer any details on the attack except to confirm that an incident happened.
"Our full attention is on the health and well being of our colleagues," said a release from Loblaw public relations.