Thursday's incident involving 200-300 students and two dozen staff members left several people injured. Most received medical treatment at the school.
Five people, including three students and two staff members, were transported to area hospitals for treatment.
Students say the melee that began as a small altercation in the lunch room may have stemmed from racial tension between Somali and African-American students.
Deputy Chief Kris Arneson says she's hopeful investigators can find out what happened from school resource officers at the scene in combination with surveillance images.
The Star Tribune reports the investigation could take weeks.
So far, no charges have been filed.
Extra officers were on hand at the high school Friday, where classes resumed as normal.
Here's cell phone video of the incident that was posted of YouTube.