Los Angeles police said they found no evidence of a shooting on the USC campus Monday after reports of gunfire prompted a lockdown and a substantial LAPD response.
"No danger to community," the Los Angeles Police Department said on Twitter after completing a search of campus buildings.
Public safety officials said evacuations were prompted by a false report that came in shortly after noon.
John Thomas, head of USC's Department of Public Safety, said in an emailed statement, "It was reported a faculty member during class falsely told her students there was an active shooter in the building."
USC police said the incident happened in a third-floor classroom in Fertitta Hall, a building of the business school.
LAPD Deputy Chief Phillip Tingirides said the female professor appeared to have some sort of "episode" and told students there was an active shooter on campus.
The woman, whose name department officials declined to release, was not booked on a criminal charge. She was detained for questioning, LAPD Officer Tony Im said.
At this point police are trying to figure out "why she did what she did," Im said.
"They're going to question her about everything," he said.
USC issued an alert telling students to shelter in place at 12:23 p.m. Within about half an hour, police had verified there was no active shooter.
Tiago Rodriguez was sitting in a business communication class in Fertitta Hall when campus public safety officers walked by and told everyone to stay inside. Minutes earlier, the professor had been going over the procedures for an active-shooter situation.
At first, Rodriguez thought it was a drill, but the mood in the class intensified quickly.
Students lined up against a wall. One student gave the professor his belt to lock the door. At one point, the people in the room piled up their desks and chairs in front of the door to form a barricade. Some students called their parents, crying.
"It totally felt like my life was in immediate danger," Rodriguez said. "It was sobering that this is the reality of the United States today."
Soon after the reports surfaced, a police helicopter circled over the campus and students sitting outside a campus food court were moved inside. Some students and employees said they sheltered in place inside classrooms and offices.
Brian Frost, an MBA student, was grabbing coffee before his 12:30 p.m. class when he saw a stampede of people sprinting out of a building.
"I just started running," he said.
Frost said he thought people were even more jittery than usual in the aftermath of the mass shooting in Las Vegas on Sunday night.
"It's one of those things where you can't not think about it," he said.
Ignacio Barron, also an MBA student, said he was in the middle of an exam when a person burst into the classroom with orders to evacuate.
Barron and his classmates quickly left Popovich Hall and waited for updates as fully armed police stood nearby.
Barron said everything happened so quickly that he didn't have a chance to be fearful. He praised the school's response.
"Everything was really well-coordinated," he said.
Officers with the school's department of public safety let students back into classrooms about 2 p.m.
In a statement released late Monday afternoon, USC Provost Michael Quick praised the rapid police response as well as students, staff and faculty for how they reacted to the alert before referring to the mass shooting in Las Vegas, which left more than 50 people dead and 500 injured.
"In the aftermath of the tragic events in Las Vegas on Sunday, we understand this is a time of stress and anxiety. We want to remind you of the counseling resources we have available," Quick wrote.
6:35 p.m.: This article was updated with additional information about the faculty member and what happened inside a classroom.
5 p.m.: This article was updated with a statement from USC Provost Michael Quick.
3:30 p.m.: This article was updated with information about a detained faculty member.
2:30 p.m.: This article was updated with information from USC public safety officials and comments from a USC student.
1:40 p.m.: This article was updated with comments from a USC student.