From what I’ve heard, a lot of the students thought the sound came from construction going on next door.
Yes, some of the TV video broadcast had a multistory concrete building in the early stages of construction in the background behind their reporters. Just concrete floors, walls and exposed wall framing. If the construction crews were setting studs with explosive nail guns, their report could be mistaken for a gunshot (and vice versa).
It is interesting that the same confusion of gunshots for construction noise was also mentioned by some of the students and staff in the aftermath of the murder-suicide at the University of Washington in Seattle - about a month ago(?).