Everyone put their hands up and then were led out of the airport terminal to the international terminal, Rosen said.
As they were led out they saw broken glass from a window that looked like it'd been shot out. Rosen left his bag behind.
"It was scary I've never experienced anything like this before," he said.
The Federal Aviation Administration said a "ground stop" was in affect for all flights heading to Los Angeles, meaning planes in any other airport in the country can't take off for the city, although some flights already in the air were allowed to land.
LAX air traffic controller Michael Foote said some flights were still being allowed to depart.
Foote said his colleagues in the control tower saw passengers spilling from the terminal onto the tarmac, "evacuating the building, getting out as fast as they could." Officers eventually corralled them.
Other travelers described a chaotic scene as airport security staff evacuated terminals, including onto to the tarmac. Hundreds of people remained gathered outside next to airplanes as authorities investigated what happened.
"People started saying there's a shooter, there's a shooter," said Natalie Morin, a senior at USC who was heading to San Francisco for a graduate school interview.
Emergency crews responded to a multi-patient incident, Fire Department spokesman Brian Humphrey said. Terminal 3 is home to Virgin America and other airlines.