“The documentation is extensive,” said police spokesman Stefan Gustafsson.
“It requires a lot of work for identify a suspect… It could take months.”
Police are receiving a steady flow of reports about disturbances from around Gothenburg.
On Thursday morning rumours circulated that young people were gathering for further clashes, but police spokeswoman Malin Sahlström said the rumours were contradictory and that there were no clear signs of further disturbances.
Plusgymnasiet, the school that was at the centre of the Instagram riots, re-opened on Thursday.