Wiz Khalifa -- Man Shot and Killed at Concert

Wiz Khalifa's concert near San Jose, CA Friday night turned deadly ... when a man was shot to death ... TMZ has learned.

The concert was at the Shoreline Amphitheatre in Mountain View, CA. We're told just after 11 last night ... shot's rang out as Wiz performed.

One source tells TMZ ... the victim was shot backstage and was hit at least 5 times.

Police told us the victim died.

Cops are investigating and looking for witnesses. We've learned there are indeed people who saw the shooting but as far as we know they have not yet spoken to cops.

The shooter is still on the loose.

10:05 AM PST: An eyewitness tells TMZ ... the gunman and the victim were backstage arguing before shots were fired. It was at the very end of the show and Wiz was still on stage. The 2 men were in a dark hallway and we're told even the surveillance video would probably be unhelpful.

The people backstage were all held by cops for 3 - 4 hours ... most were interviewed.