A gunman opened fire at a bar in Thousand Oaks, California, killing at least 12 people during a college night celebration, including a deputy sheriff who was planning to retire next year. The gunman, identified as 28-year-old Ian Long, is also dead, authorities said.
Police were called to the Borderline Bar & Grill in the Ventura County suburb, 40 miles northwest of downtown Los Angeles, late Wednesday Pacific time.
"It's a horrific scene in there," Ventura County Sheriff Geoff Dean told a news conference in the parking lot of the Borderline Bar & Grill. "There's blood everywhere."
The massacre was the latest mass shooting in the United States and came less than two weeks after a gunman killed 11 people at a synagogue in Pittsburgh.
Dean said multiple 911 calls were received at 11:20 p.m. of shots being fired. "Approximately three minutes later, a highway patrol officer and a deputy made entry. Upon going through ... (the) sheriff''s sergeant was shot multiple times with gunfire," he said.
The motive for the latest mass shooting was not known. The gunman was tall and wearing all black with a hood over his head and his face partly covered, witnesses told TV stations at the scene. He first fired on a person working the door, then appeared to open fire at random at the people inside, they said.
Sarah Rose DeSon told ABC's "Good Morning America" that she saw the shooter draw his gun.
"I dropped to the floor," she said. "A friend yelled `Everybody down!' We were hiding behind tables trying to keep ourselves covered."
The 12 dead included Deputy Sheriff Ron Helus, a 29-year veteran of the agency who was looking to retire next year, Dean said, choking back tears while talking about his longtime friend.
"Ron was a hardworking, dedicated sheriff's sergeant who was totally committed," Dean said, "and tonight, as I told his wife, he died a hero because he went in to save lives."
President Donald Trump tweeted on Thursday that he has been "fully briefed" about the latest massacre.
The shooting took place during Borderline's college night, and hundreds of people were believed to be inside. A number of universities have issued statements about the shooting.
"The University has received reports that several Pepperdine students were at Borderline Bar and Grill in Thousand Oaks this evening when a gunman reportedly entered the location and began shooting," Pepperdine University said. "At this time, the University is working to identify and provide support to those students."
The Associated Students of CLU Government, a student organization that serves as the united voice of the student body of California Lutheran University, took to Facebook to show solidarity to the those affected by the shooting.
"Our hearts go out to all who were affected tonight by the shooting at Borderline. We are here for you in this tragic time, whatever the need may be. Stay safe," the Facebook post read.
Social media was flooded with reports and reactions as the shooting played out. A local media reporter Jeremy Childs tweeted photos and videos from the incident.
The suspected gunman lived in the Newbury Park section of Thousand Oaks, five miles from the scene of the shooting.
Correction: This story was revised to update with authorities giving the suspect's age as 28.
—CNBC's Jeff Daniels, Marty Steinberg and The Associated Press contributed to this article.