A call to police indicated the man was spotted inside an elevator on the 11th floor of the building. That led a huge response in manpower, including university police, Chicago Police Department officers and federal agents, who arrived in SWAT gear. During the search, students and staff waited inside their rooms. Many hid under desks.
"I just think it's really crazy that there is someone in there," said one person on the campus before the lockdown ended. "It seems like they have the situation under control. I'm not particularly terrified."
The Rubloff Building is home to NU's law and medical schools. The university sent text messages and automated phone calls to faculty.
"I got an automated phone call at home, something over an hour ago indicating that there was a man in the building with a gun.," said Andrew Koppelman, Northwestern law professor.
Three hours later, police allowed students to leave. Many now wonder if there ever was a gunman.
"I'm concerned it might have been a hoax," said Robert Peachey, Northwestern law student.
A Northwestern official said she had no information on that possibility and said the only call came from a credible source, who is not a student.
"This is all I have right now to give you guys," said Marla Paul, Northwestern spokesperson.
Officials said university police are still investigating.