I’m thinking about the killing of Martin Luther King, and the Rodney King verdict, reporting of which would have been expected to cause widespread unrest at the time - and did - and in neither case was there an effective attempt to cover things up.
It strikes me that a cover-up in the case of panic would be the kind of thing the authorities would only undertake if they were absolutely certain of keeping a tight lid on it for decades, without the possibility of an early and credible leak. E.g. some mishap at NORAD, or a mistake with a B-52’s nuclear payload during peacetime, or an incident with a reactor on board a US aircraft carrier whilst in a friendly port. Something like that. The details would come out thirty years later, by which time no-one would care.
For any case where the authorities could not guarantee absolute secrecy I surmise that the consequences of a failed cover-up would actually make things worse. I remember NASA came in for a lot of stick after the Challenger disaster because it took them several hours to formally acknowledge that the orbiter had been destroyed and the crew killed even thought it was obvious from the television footage that something had gone badly wrong. That wasn’t a deliberate attempt to cover things up or indeed a cover-up at all, it was institutional inertia, but it made the organisation seem even more inept than otherwise.
Mind you, as a mental exercise, suppose the Rodney King trial had been held behind closed doors, and the verdict had been given out only to those present? I assume this would have been impossible under US law at the time; I’m not American, and I’m not a lawyer. The riots might not have happened if news of the verdict had been confused, obfuscated with legal mumbo-jumbo, and delivered a week later. And e.g. the French tried to cover up the killing of a Greenpeace activist when their secret service men sunk the Rainbow Warrior - which failed, but again the outrage was diluted and the incident is mostly forgotten nowadays. There were no mobs throwing bottles at French embassies, for example, because the outrage was diluted. Dilution rather than a solid wall of silence seems to be the key.
Here’s a real-world example, in which then-PM Winston Churchill ordered the cover-up of UFO sightings in the UK in order to avoid unrest:
*Nick Pope, who used to investigate UFO sightings for the MoD, said: "The interesting thing is that most of the UFO files from that period have been destroyed.
"But what happened is that a scientist whose grandfather was one of his [Churchill’s] bodyguards, said look, Churchill and Eisenhower got together to cover up this phenomenal UFO sighting, that was witnessed by an RAF crew on their way back from a bombing raid.
“The reason apparently was because Churchill believed it would cause mass panic and it would shatter people’s religious views.” *
Outside the context of UFO sightings, in the real world of riots and racist murder and religious strife etc, there are persistent allegations that the authorities and the media were unwilling to investigate and report on the Rochdale sex trafficking ring for reasons that are obvious from the Wikipedia article. Apart from that, I suspect the authorities have concluded that most people are apathetic, impotent, and ineffectual. What could cause a mass panic that would threaten the government of the UK or the US, that would not blow away with a few cold nights; or that the government’s security forces could not deal with? Who, after all, speaks today of the Occupy movement?