Lewis and Clark Community College campus went on lockdown Monday after a gun was spotted in a classroom.
Officials said a student saw the firearm and reported it to a family member.
The college sent out a text message to students about a gun on campus.
Official said the gun belonged to an off-duty Alton police officer who is a student at the college.
Illinois law does allow any officer, active, off duty or retired to carry a gun on a college campus.
Madison County Sheriff Robert Hertz said law enforcement students are not required to report that they are on campus.
School officials said the officer did tell his instructor about the gun, but no one else. They also said the student always brought his gun to class.
Hertz said the female student who saw the gun did the right thing in alerting the school.
Officials said the officer will be back in class this week and will be allowed to have his gun.
Information from KMOV.com contributed to this report.