The University of Houston campus is on alert after a third student is threatened by an armed robber.
There have been three similar incidents on or near the campus in less than a week.
University police are patrolling the campus Friday after the third incident took place Thursday night in which police say an armed man tried to rob a student.
Since the student had no money, the robber hit the victim in the jaw and ran away.
Police are warning students to stay in groups and not to walk around the campus alone.
"I think everybody just kind of tries to stay around somebody if you don't always walk with a certain person. It's a big campus, you don't really know everybody you're parking around so everybody kind of tries to stay around people they know," said University of Houston student Courtney Mills.
An armed man stole cell phones and other personal property in two other robberies that took place.
Police say they have not determined whether the robbery attempts are related.