Cover-up

A cover-up is an attempt, whether successful or not, to conceal evidence of wrongdoing, error, incompetence or other embarrassing information. In a passive cover-up, information is simply not provided; in an active cover-up, deception is used.
The expression is usually applied to people in positions of authority who abuse power to avoid or silence criticism or to deflect guilt of wrongdoing. Perpetrators of a cover-up (initiators or their allies) may be responsible for a misdeed, a breach of trust or duty, or a crime.
While the terms are often used interchangeably, cover-up involves withholding incriminatory evidence, while whitewash involves releasing misleading evidence. See also misprision.
A cover-up involving multiple parties is a type of conspiracy.
Modern usage.
When a scandal breaks, the discovery of an attempt to cover up the truth is often regarded as even more reprehensible than the original deeds.
The mildest case, not quite a cover-up, is simply to release news which could be embarrassing but is not important enough to guarantee attention, at a time when other news is dominating the headlines, or immediately before a holiday or weekend.
Initially a cover-up may require little effort; it will be carried out by those closely involved with the misdeed. Once some hint of the hidden matter starts to become known, the cover-up gradually draws all the top leadership, at least, of an organization into complicity in covering up a misdeed or even crime that may have originally been committed by a few of its members acting independently. This may be regarded as tacit approval of that behaviour.
It is likely that some cover-ups are successful, although by definition this cannot be confirmed. Many fail, however, as more and more people are drawn in and the possibility of exposure makes potential accomplices fearful of supporting the cover-up and as loose ends that may never normally have been noticed start to stand out. As it spreads, the cover-up itself creates yet more suspicious circumstances.
The original misdeed being covered may be relatively minor, such as the "third-rate burglary" which started the Watergate scandal, but the cover-up adds so many additional crimes (obstruction of justice, perjury, payoffs and bribes, in some cases suspicious suicides or outright murder) that the cover-up becomes much more serious than the original crime. This gave phrase, "it's not the crime, it's the cover-up".
Cover-ups do not necessarily require the active manipulation of facts or circumstances. Arguably the most common form of cover-up is one of non-action. It is the conscious failure to release incriminating information by a third party. This "passive cover-up" is often justified by the motive of not wanting to embarrass the culprit or expose them to criminal prosecution or even the belief that the cover-up is justified by protecting the greater community from scandal. Yet, because of the passive cover-up, the misdeed often goes undiscovered and results in harm to others ensuing from its failure to be discovered. 
Real cover-ups are common enough, but any event which is not completely clear is likely to give rise to a thicket of conspiracy theories alleging covering up of sometimes the weirdest and most unlikely conspiracies.
"Snowjob" is an American and Canadian colloquialism for a deception or a cover-up; for example, Helen Gahagan Douglas described the Nixon Administration as "the greatest snow job in history".
Typology.
The following list is considered to be a typology since those who engage in cover-ups tend to use many of the same methods of hiding the truth and defending themselves. This list was compiled from famous cover-ups such as Watergate Scandal, Iran-Contra Affair, My Lai Massacre, "Pentagon Papers", the cover-up of corruption in New York City under Boss Tweed (William M. Tweed and Tammany Hall) in the late 1800s, and the tobacco industry cover-up of the health hazards of smoking. The methods in actual cover-ups tend to follow the general order of the list below.
Alleged covers-up.
Conspiracies to cover up the facts of a number of prominent events have been alleged in the following cases: