Las Vegas Metropolitan Police reported shots were fired near the Mandalay Bay casino a little after 10:30 pm Pacific Time Sunday evening.
The incident took place during the Route 91 outdoor country music festival.
Police say one suspect has been killed, but reports of other shootings elsewhere on the Las Vegas Strip are also being investigated.
Concertgoers reported hearing automatic gunfire, and posted videos of such on social media.
Local television stations report at least 70 people have been taken to area hospitals, with one having to turn patients away due to being at capacity.