What's the legal authority in this case??

Hunter Nash, is an independent journalist with a popular blog and YouTube channel.While live-streaming a large political protest in O'Fallon, Missouri, Nash was caught up in a "kettling" incident when lines of heavily armed police in full riot gear surrounded protesters and corralled them into an intersection.More than 150 people, including Nash, were held in that location while police processed arrests and then transported individuals to headquarters.During this time, one officer violently wrenched Nash's arm behind his back in an attempt to stop him from recording the events with his cell phone.When Nash protested and asserted his status as a journalist, he was zip tied, sprayed with pepper spray, and forced to sit on the pavement with other detainees for several hours.