A 3-year-old boy has been shot and Robert Brownkilled while in the car with his mother and brother in Cleveland, authorities said.

An adult man -- who was not in the car -- was also shot in the Thursday afternoon incident, Cleveland police said. He suffered non-life-threatening injuries, according to police.

Several persons of interest have been detained, police said Friday, noting that the investigation is ongoing.

The 3-year-old's "heartbreaking" killing "is another reminder that we must come together as a city, state and nation to address this violence and its root causes," Cleveland Mayor Justin Bibb said in a statement.

Last year, at least 6,152 children were killed or injured in shootings in the U.S., according to the Gun Violence Archive.

"No words can adequately express the pain and the sorrow of this tragedy. In the face of this loss, I ask the community to come together to support this family and to reaffirm our commitment to a safer city," Bibb said. "We will not rest until those responsible are held accountable and brought to justice."