U of I has templates ready to go for certain emergency situations. An employee was updating one of those templates when, instead of saving it, he sent it to about 40,000 students and staff.
"Active shooter at building name/intersection. Escape area if safe to do so or shield/secure your location," it read.
Within a few minutes, an e-mail was sent out explaining the mistake.
U of I is on spring break.
"It's safe to say (30,000) to 40,000" people received the warning by e-mail, text message or other means, university spokeswoman Robin Kaler said.
Students, faculty and others have to sign up to receive Illini Alert messages. The system was created in 2007 after a student at Virginia Tech University killed 33 people in a campus shooting spree, including himself.
The Associated Press contributed to this report.