Thus, following the shooting, the police had the right to not only arrest any person they observed shooting weapons at others, they would have the right to conduct a protective sweep of not only the outside of the restaurant, but the inside of the restaurant for persons they believed would be a continuing danger. Any person found to be in possession of a firearm would be in violation of the law if the restaurant, Twin Peaks, sells alcohol and the alcohol accounts for 51% of its revenue. Possessing a firearm, even with a conceal and carry permit, is prohibited on the premises (including the parking lot) of such locations. Any customer possessing a firearm inside the restaurant without a conceal and carry permit would also be subject to arrest.
However, the jump to arresting everybody present is where the Constitutionality of the arrests is questionable. Police would, after a protective sweep, be able to pat down suspicions persons for “officer safety” if they have reason to suspect they might be armed and a danger to the officers. Wearing a jacket matching those who were involved in the shootout probably gets officers to this standard, and it is probably not unreasonable to pat down every person reasonably suspected to have affiliation with those involved in the shootout. If they are in possession of a firearm or other illegal items, they can be arrested for such and these are also felony level charges. But probable cause of conspiracy to commit capital murder is a very high standard that is probably only reachable with a few suspects.