Four people were shot dead on a commuter train in Chicago, Illinois, police said, adding that a suspect had been arrested.
Police said they arrived at the Forest Park train station where they found the four victims.
"The victims were all passengers on this train," Forest Park Deputy Chief Christopher Chin said at a press conference, adding that the suspect was identified using surveillance footage and arrested.
"This appears to be an isolated incident and does not pose an immediate threat to the community," police said in a statement.
The United States has one of the highest rates of gun deaths in developed countries, a phenomenon attributed to the prevalence of individual weapons. One in three adults owns at least one gun, and one in two American adults lives in a home with a gun.
In June, Vivek Murthy, the director of public health in the United States, declared gun violence a "public health crisis," calling for broad gun controls.
