An initial note appeared on the school's website at 9:12 p.m., warning of an "active threat" outside Wieboldt Hall in the Streeterville neighborhood.
The note added that Chicago and Northwestern police were responding on Chicago Avenue near North DuSable Lake Shore Drive.
Another note, posted at 9:32 p.m., said the situation "has been secured and is all clear."
"Assailants are in custody. It is not known whether anyone was injured," the note continued. However, Chicago police said late Monday night that Northwestern police may have detained some people, but must have released them.
Police said officers responded to a shots fired call in the area and spoke to a witness, who said there was a large fight in the street involving several people.
One person, identified only as female, was seen removing a handgun and fired one time, police said. The crowd then fled the scene, and CPD did not take anyone into custody.
Police said no injuries were reported and Area Three detectives are investigating.
The Chicago Fire Department said they have not gotten reports of anyone shot in this area.
Authorities did not immediately provide further information about the situation.