Do the people who were assaulted have legal recourse against the city?
Probably not, unless it can be proven that the police failed in their duty to know something like this would happen.
From Black’s, res ipsa loquitur is a: “Rebuttable presumption or inference that defendant was negligent, which arises upon proof that instrumentality causing injury was in defendant’s exclusive control, and that the accident was one which ordinarily does not happen in absence of negligence. Res ipsa loquitur is rule of evidence whereby negligence of alleged wrongdoer may be inferred from mere fact that accident happened provided character of accident and circumstances attending it lead reasonably to belief that in absence of negligence it would not have occurred and that thing which caused injury is shown to have been under management and control of alleged wrongdoer. … When a thing which causes injury, without fault of injured person, is shown to be under exclusive control of defendant, and injury is such as in ordinary course of things does not occur if the one having such control used proper care, it affords reasonable evidence, in absence of an explanation, that injury arose from defendant’s want of care.” Black’s Law Dictionary 6th, 1305.