Local police confirmed to PEOPLE that the shooting occurred around 11 p.m. backstage at Shoreline Amphitheater, where Khalifa, 26, headlined a sold-out show for about 22,000 people.
Mountain View Police spokeswoman Shino Tanaka said authorities received multiple calls about the incident and that a 38-year-old man was transported to a local hospital, where he later died of multiple gunshot wounds.
Police are investigating the incident and have not released the victim's identity. A source told TMZ that the victim was shot five times. The shooter remains at large.
Tanaka described the shooting suspect as a black male in his mid-20s with short hair, between 5'6" and 5'8" tall, and who was believed to be wearing all black, white pants and a red baseball cap. Police are also asking witnesses who may have video or photos of the backstage area to come forward.
Khalifa is currently on a national tour alongside Young Jeezy. He is nominated for best hip-hop video at Sunday night's MTV Video Music Awards for "We Dem Boyz."
The rapper released a statement on Twitter calling the incident "a senseless tragedy."
"Violence is never the answer," he added. "My prayers go out to the victim's family."
This was a senseless tragedy. Violence is never the answer. My prayers go out to the victim’s family.— We Dem Boyz (@wizkhalifa) August 23, 2014