At least 200 people were reportedly inside the Borderline Bar and Grill, which was hosting a university student night, at the time of the attack.
Officials say the suspect, who has not been identified, was found dead inside.
They have not yet confirmed any motive for the shooting.
A college country music night was under way when the suspect, apparently dressed in black, entered the bar and opened fire.
Reports say the suspect may have used smoke grenades and at least one weapon, described as a semi-automatic handgun by witnesses.
A injured man told local television channel KTLA: “We just dropped to the ground, we heard a lot of screaming. My friend is the DJ so she cut off the music, we just heard a lot of mayhem.”
Another witness from inside, Teylor Whittler, said: “I was on the dance floor and I heard the gunshots, so I looked back and then all of a sudden everyone screamed: Get Down!”
“It was a huge panic, everyone got up, I was trampled, I was kind of left on the floor until some guy came behind me and grabbed me and dragged me out.”
People reportedly escaped the bar by using chairs to break windows, while others reportedly sheltered inside the venue’s toilets.