Two rival motorcycle clubs were "possibly" involved in the violence at the Colorado Motorcycle Expo, police Chief Robert White said at a press conference on Saturday night.
The Denver Post quoted a witness, who didn't want to be identified because of fear of retaliation, as saying the violence involved rival biker gangs, CNN reports.
Video taken after the shooting showed police talking to groups of men wearing jackets with motorcycle club patches.
White said about 10,000 people were inside the National Western Complex hall at 12:45 pm (2:45 pm ET) when shots were fired.
Investigators think two or more people exchanged gunfire, he said, but the cause of the shooting is still being investigated.
One person was fatally shot and three were wounded by gunfire, White said. One person was stabbed and three other people suffered injuries probably caused in a fight, but not by weapons, he said.
No arrests have been made but people are being questioned.
"Whether it was motorcycle gang-related or motorcycle club-related, that is yet to be determined," the chief said.
The 38th annual expo for Sunday has been canceled, according to the city, but the Arenacross event will go on Saturday night and Sunday afternoon at the National Western Complex. Arenacross has no relation to the expo, the release said.
White said seven people were injured. Earlier in the day, a Denver Health Medical Center doctor said six people were hurt, with three in critical condition and three in stable condition. Before that, Denver Health had tweeted that nine people had been taken to area hospitals.