The police are investigating a possible breach by protesters of a Parades Commission ruling after violence during an Orange Order parade in Belfast.
Water cannon were deployed by police during clashes
Eighteen police officers and 11 others were injured when trouble erupted at Friday's Tour of the North parade.
Missiles were thrown by nationalist protesters as the parade passed a flashpoint area at Ardoyne. Three people were arrested.
Gary White of the PSNI said police did their best to deal with the situation.
"We pushed the parade through. We had a fairly significant operation in place... in order to get both the parade and the supporters through," said Superintendent White.
"We had to take a balanced judgement. Had we have held the supporters where they were.. would that violence still have happened? I think it would and perhaps it could have spread to other areas.
"So with the operation in place, the tactical police decision was to ensure that we complied with the parades commission determination - and pushed the supporters through as well."
Trouble flared at about 2100 BST on Friday as three lodges, followed by supporters, went past Ardoyne shops on the Crumlin Road on the return leg of their journey.
Nationalists had gathered to protest against the march.
Marchers had been separated from the protesters by a corridor of about 60 Land Rovers and police in riot gear, but were pelted with missiles, including bottles, bricks and golf balls.
There were further clashes between police and protesters and two water cannon were deployed by police in an attempt to regain order.
Six petrol bombs were then thrown. A teenager's arm was broken during the trouble.
The police are to examine CCTV footage from the scene. They said they would be intent on bringing law breakers before the courts.
Sinn Fein's Gerry Kelly said the blame for the trouble lay with the original ruling by the Parades Commission.
"It has been a complete disaster from the decision," he said.
"We spent all year arguing that the Parades Commission should have the ability to deal with supporters as well," he said.
However, North Belfast MP Nigel Dodds, of the DUP, said the fault lay with the protesters.
"The determination allowed for a peaceful protest. The protest was violent. Orange brethren, bandspeople and supporters and everybody else followed the determination to the letter.
"Even when they were under attack, nobody responded. They didn't flaunt anything, they didn't have anything provocative. They were attacked viciously."
SDLP assembly member Alban Maginness said the trouble did not auger well for the coming marching season.
"There have been a series of incidents here, and I must say, a very, very unfortunate lead into the marching season," he said.
"This does not bode well. It has done nothing for community relations."
The march was the first to be affected by an extension of the law governing the behaviour of parade supporters.
It gave police wider powers to control the movement and behaviour of parade followers at flashpoint areas.
The Tour of the North is among the first of a series of parades by Protestant Orangemen which culminates in the biggest demonstrations on 12 July.
A ruling by the Parades Commission had restricted nationalist protesters to the footpath outside the Ardoyne shops and loyalists supporters also face restrictions, following conflict at a parade last July.