Security personnel stand guard June 1 at the University of California at Los Angeles. (Robyn Beck/AFP via Getty Images)

Police ran past in the middle of a presentation by students in the graduate engineering class on biological treatment at the University of California at Los Angeles. Then a text message from the university urged everyone to shelter in place.

The student who was giving a presentation, in the building where the shooting happened, stopped to answer texts from his mother asking whether he was okay. He answered, apologized and continued.

Then “a SWAT team was running down the hallway, telling everyone to lock their doors,” said Christine Zhang, a student in the class. “But UCLA doors don’t really have a lock. So we had to kind of improvise.”

They heard a loud sound, a bang, she said. They saw people running in and out. “We took turns holding the door — it was pretty scary.”

Everyone was wondering whether they were going to survive, she said.

And in an odd way — as extraordinary, as volatile, as unpredictable and unknowable as the circumstances were — “we all knew what to do,” she said.

“We all knew what to do if there were an active shooter.”

Two people were killed in a likely murder-suicide in an office on the UCLA campus, LAPD Chief Charlie Beck said on June 1. Students and staff are slowly being released from lockdown. (Reuters)

After Virginia Tech, after Sandy Hook, after Umpqua, after countless scares and threats this past year, it has come to this: Students are ready for senseless violence.

“It was very systematic, very calm,” said Daphne Ying, who hid with classmates in another part of campus after the alert. “It was a step-by-step thing. Gotta shut off the lights. Lock the doors. Sit toward the front of the classroom.”

They contacted their families. They quietly shared information as they got updates from friends. “The only way I knew people were really scared is we were all whispering very quietly. If  anyone raised their voice to a slightly louder whisper, that person would immediately get shushed, immediately.”

Everyone was on their phones, trying to figure out what was happening. Reports were wildly different, from the media and from people on campus. “We didn’t know what to believe,” Zhang said.

They took photos of SWAT teams outside the windows.

As they waited, they wished they had food, chargers, bathrooms.

And they traded rumors. Several students said they heard there were four shooters, working together, on multiple parts of campus. Shots were reported here, there.

Garrett Gregory, a sophomore who grew up in the District, was sitting in a lecture when people were distracted by alerts on their phones. A girl in his class raised her hand and said, “The shooter is in Kerckhoff,” a coffeehouse nearby. He took shelter in a lab in the basement crowded with students all exchanging information and misinformation. It was all confusion. Many thought it was a terrorist attack. And he was getting messages from one of his best friends ,who was hiding in her dorm room, terrified by rumors that a shooter was in her building. “It was pandemonium,” he said.

On the north side of campus, students saw reports of a shooter shortly after 10. But then, about 10:20 a.m. local time, “We heard this very, very loud shot. It echoed. That’s when we got worried: If the shooting is all the way on south campus how are we hearing this?” Ying said. “I got a message on my phone: The shooter has moved to north campus. We stopped class. Shut off all the lights. Realized the door is unlocked and opens into the hallway.”

Her classmates improvised: They took the cord from a projector and wrapped it around a chair bolted down in the room. And they set up a table to block the door, so that if the cord failed it would block an intruder’s view of the room — and fall on whoever was trying to get in.

Some students stood with their hands on the doorknob, waiting.

Gizmodo noted people locked down in the building were engineering barricades for doors without locks.

“1 whole wall of my classroom is opaque glass … ” Another tweet read: “So scared right now.”

Everyone just waited in silence, in the dark, Ying said. One wall of the classroom had a frosted-glass window, so they were staying away from that to avoid any sign that people were in the room, she said. But then they heard helicopters, and heard noises in the hallway.

They were wondering if it might be a terrorist attack.

They saw shadowy silhouettes of four people walking in the hall. They could tell they were armored, and had weapons. The fourth person put his hand on the doorknob, trying to get it open, she said. There was no indication that they were police, she said.

“That was definitely terrifying,” she said. “That was the scariest moment. We didn’t know who they were.”

When the lockdown was lifted, students emerged, bleary-eyed and shell-shocked, Gregory said. “There was this mass exodus of students leaving campus presided over by SWAT teams and police helicopters circling overhead.”

Ying still had a pit in her stomach Wednesday evening. She wanted answers about what had happened, and why.

And yet, for all the fear, she said, “I just feel like it’s come to a point where we’ve seen so many of these instances on the news, it’s become a part of daily life.

“This is just something that happens now. This is just a way of life.”