The shooter, whom police shot and killed at the scene, was a 45-year-old employee of a manufacturing plant in suburban Chicago.
A terrorism inquiry is launched into the incident near a famous Christmas market. The gunman has been identified and a manhunt is underway.
Killed were a doctor, a pharmacy resident, a policeman and the gunman himself. A quarrel between former lovers was reportedly involved.
Psychologists, however, are still debating the links between PTSD and violent behavior.
The massacre is the deadliest in the U.S. since 17 classmates and teachers were gunned down at a Parkland, Fla., school nine months ago.
The gunman, who is in custody, is believed to have posted anti-Semitic slurs and rhetoric on social media.
Manuel Oliver lost his son in February's school shooting in Florida. He has now made it his life's mission to control guns in the hope that no other parent will lose a child in the way he did.
A man with a handgun shoots into restaurants and cafes as he walks along a Toronto street before dying after an exchange of gunfire with police.
The American Medical Association says gun violence is as menacing as a lethal infectious disease.