The shooting took place at the Great Lakes Drag away in Union Grove, Wisconsin, Kenosha County Sheriff David Beth told reporters at the track in news footage carried on the FOX6 News website.
Several thousand people were at the track at the time, which was evacuated. About 100 police officers responded to the shootings, interviewing witnesses, combing for evidence and helping with the investigation, Reuters reported.
No suspects were in custody, authorities said, and no motive had been determined.
"We're not aware of any danger to the community," Beth said.
The victims, all men in their 20s or early 30s, were from Illinois. Their identities were not released.
The men were standing near a food vendor when a gunman approached and shot them at what appeared to have been point-blank range, Beth told reporters.
Two friends who were with the victims "had no idea what caused this," he said.
"We're not aware of any altercations that might have happened earlier," Beth said.