Police responded to a shooting outside Colorado's Red Rock Amphitheater late Thursday night by forcing thousands of concert goers through a mandatory checkpoint.

The shooting, which left three people with non-life threatening injuries, occurred in the parking lot following performances by rappers Nas, Flying Lotus and Schoolboy Q.

"I'm backstage; we're on lockdown waiting to leave," concert promoter Faithon Lucas told the Denver Post.

Attendees of the concert took to Twitter to air their concerns as law enforcement began searching vehicles and demanding ID from every one attempting to leave.