The shooting outside El Camaron Gigante Mariscos & Steakhouse was an "isolated incident," and authorities believe more than one person fired shots, Glendale Police Department spokesperson Jose Santiago said.
Between 200 and 300 people were in attendance at the outdoor venue when the shooting broke out.
Santiago said multiple people were being questioned but no arrests had been made and police are still searching for other possible suspects.
"Our detectives are trying to narrow down the scope of how many shooters we are dealing with," Santiago said, adding that detectives were still collecting shell casings outside the restaurant.
The dead included a 17-year-old boy and his 21-year-old brother, as well as a 29-year-old man, Santiago said.
Police have not released their names.
The wounded included a 16-year-old boy who was critically injured and required surgery overnight, but Santiago said he was expected to survive. Other people between the ages of 20 and 23 were injured.
In a statement, the restaurant said it was co-operating with the police investigation while trying to make sense of a "heartbreaking event."
"Our hearts go out to the victims, their families, and all affected by this senseless violence," the statement said.
Social media videos shared by the restaurant before the shooting showed people dancing near a stage with a DJ booth.
Santiago said event organisers removed a group of people from the venue after they started fighting near the stage. They began shooting at each other just outside of the event, Santiago said.
Bystander Lupe Rodriguez said he ran to safety when he heard the gunshots. He was shaken, but said he was grateful that he and his friends survived.
"There was a man on the ground, and it didn't look like he made it," Rodriguez said. "His father was yelling out his name."
25°C