About 50 people were involved in the gun salute at Uthenthawai campus in Pathumwan district on Tuesday morning, according to police.
The shots were reportedly fired skyward to celebrate the 88th anniversary of the founding of the campus.
Pol Col Pansa Amarapitak, chief of Pathumwan station, said on Thursday that investigators had checked surveillance camera footage and questioned witnesses.
They concluded that about 50 people gathered at the campus of Rajamangala University of Technology Tawan-ok on Phaya Thai Road on Tuesday. Some were women.
Police were in the process of identifying them all. About 10 were clearly seen involved in firing shots into the air from handguns, he said.
When identification was complete, police would ask teachers at the campus to confirm if they were students or outsiders. If campus management had not given permission for the gathering, the outsiders would be charged with trespassing, he said.
According to Pol Col Pansa, the 50 people first gathered at a building near Phaya Thai Road, then marched to a football field on the campus, where they fired gunshots skywards at 7.22am before dispersing.
He said the participants had broken the law by illegally having firearms and ammunition, firing guns and carrying weapons in a public place without a sound cause.
Police launched an investigation after a video of the gun salute went viral on social media on Wednesday.
The same day, police searched the campus and found a variety of knives, a small amount of stashed handgun ammunition, and some spent cartridges.