- Who revealed first that Hillary Clinton has used her family's private email server for official communication?
- What was Clinton's first reaction? Did she admit the fact immediately? Did she deny it?
- Is it (using private email server for official communication) considered illegal in the US?
Who first revealed that Hillary Clinton has used her family's private email server for official communications?
Hillary Clinton's private email server was discovered through the official investigation into the 2012 attack on the US embassy in Benghazi, Libya which resulted in the deaths of 4 Americans, including the US ambassador to Libya.
What was first reaction of Clinton? Did she accepted the fact immediately? Did she deny it?
Secretary Clinton admitted that she used a private email server (which is not illegal for personal use), but denied that she ever used it for official State Department business. She also stated that she did not communicate classified information with it. Both of these statements were later shown to be false.
Is it (using private email server for official communications) considered illegal in US?
Yes. The State Department in particular handles diplomatic relations with foreign countries and therefore deals with both classified and highly sensitive and information. All Federal agencies have policies regarding the appropriate channels for official communication for the purposes of security and oversight. There are also additional, strict laws USC 18, Section 793 that govern the appropriate handling of national defense information, making the disclosure of that information to unauthorized people or in an unauthorized way a Federal crime.
During his testimony to Congress, FBI director James Comey stated that secretary Clinton was found to have violated these laws, calling her actions "extremely careless". Despite this, however, the FBI has declined to bring formal charges against her because they couldn't find "specific intent" to mishandle the information. This is controversial because subsection(f) makes it a crime to exhibit gross negligence in handling classified information — regardless of intent. It states that anyone who, "(1) through gross negligence permits [information regarding national defense] to be removed from its proper place of custody ... or (2) having knowledge that the same has been illegally removed ... and fails to make prompt report of such loss"
Thanks to @DrunkCynic for the citation of the law violated.