News conference on shooting at a nightclub in Thousand Oaks, California

Twelve people were killed, and multiple people were injured after a mass shooting at a Southern California bar late Wednesday.

About 40 miles west of Los Angeles, a gunman opened fire inside a country-western dance bar in Thousand Oaks, California. At least 11 victims were killed at the scene; a deputy later died from his injuries, said Ventura County Sheriff Geoff Dean. The gunman, later identified by police as Ian David Long, was found dead inside.

The bar was hosting an event for college students at the time.

"There is blood everywhere," Dean said Thursday morning.

This is the deadliest mass shooting in the United States since 17 students and teachers were killed at a Parkland, Florida, school in February.

Here's what we know:

What happened?

A gunman dressed in black, later identified by police as Long, 28, began firing at Borderline Bar & Grill around 11:20 p.m. Wednesday, authorities said.

Sheriff Sgt. Ron Helus was among the first to arrive on the scene, entering the bar with a California Highway Patrol officer. Helus exchanged gunfire with the suspect before being shot multiple times. He later died at a hospital.

After spraying bullets for almost three minutes and killing 11 other victims, the gunman then went into an office near the front of the bar where he was found dead, apparently killing himself, police said.