A false fire alarm caused an evacuation at Hayden Library on Sunday night.
Occupants streamed out of the building when the alarm went off, leaving the building empty until officials arrived about five minutes later.
It was not immediately clear who or what triggered the alarm, library officials said.
The Tempe Fire Department swept the building and did not find any smoke or fire, but noticed a "slight odor" in the air, a fire department spokeswoman said.
Fire officials then reset the alarms and left the scene in control of ASU police, who allowed students to re-enter the building after a few minutes, an ASU police spokesman said.
The false alarm came at an inconvenient time, with many students preparing for finals week:
Fire alarm in Hayden Library during finals. Someone's idea of a cruel joke. Watch as hundreds of students pour out of the library.— Zach Josephson (@zachjosephson) April 27, 2015
Everything appeared to have returned to normal a couple hours later, with students hard at work throughout the building.
State Press photographer Shiva Balasubramanian contributed to this report.