Four employees died instantaneously and a further three died later from their wounds, including the gunman, according to the Cook County Medical Examiner’s Office on Wednesday.
It is not yet known if the ex-employee was killed by officers or shot himself, police spokesman Pat Camden said.
Acting Police Superintendent Phil Cline said the gunman had been fired from the company several months ago for having been "a poor employee ... late to work ... absent from work ... with poor behaviour on the job."
Authorities received an early morning call informing them someone had been shot.
Police arrived at the Windy City Core Supply Inc within five minutes of the phone call.
All buildings within a block of the area were evacuated and police initially tried to negotiate with the man but he refused, Camden said.
Cline said the gunman had a record of 12 arrests, including two involving a gun, two for assault, four for domestic battery and the rest assorted traffic incidents.
It was the nation's deadliest workplace shooting since 8 July, when Doug Williams gunned down 14 co-workers, killing six, at a Lockheed Martin aircraft parts plant in Mississippi - before taking his own life.
In the Chicago area, William Baker killed four people and himself at a Navistar International engine plant in suburban Melrose Park in February 2001.