The controversy surrounding Clinton's email practices again burst into public view in August 2015 after the inspector general for the intelligence community revealed that two of the thousands of emails held by Clinton contained top-secret information.

Clinton first acknowledged that her use of a private email system while helming the US State Department was a "mistake" in September, six months after her practice was disclosed by a Republican-controlled Congressional committee which investigated events leading up to the 2012 Benghazi attacks that claimed four American lives, including the US ambassador to Libya.

The FBI's recommendation on Tuesday immediately drew criticism from Republicans, with House Speaker Paul Ryan questioning whether "damage is being done to the rule of law" in the case.

"No one should be above the law," said Ryan in a statement. "Declining to prosecute Secretary Clinton for recklessly mishandling and transmitting national security information will set a terrible precedent."

Comey has in the first place defended the FBI investigation as being done "honestly, competently and independently."

"I know there were many opinions from people who were not part of the investigation, including people in government. But none of that mattered to us," said Comey. "No outside influences of any kind was brought to bear."

Early this year, US President Barack Obama weighed in on Clinton's intent of using a private email setup during her stint at the State Department.

He told Fox News in an interview that he believed that Clinton did not intentionally endanger national security in her handling of classified information.

The remarks immediately raised concerns that the federal investigators could be swayed by the president's view on the issue.

The FBI announcement came just hours before Clinton's first joint campaign appearance with President Obama and only about a week after former US President Bill Clinton had a private meeting with Attorney General Loretta Lynch.

The 30-minute meeting on June 27 at the Phoenix International Airport in Arizona, which Lynch described as primarily social, raised concerns about a potential conflict of interests.

According to ABC News who first disclosed the meeting, Mr. Clinton waited for Lynch's arrival after being notified that she would be arriving at the same airport. Mr. Clinton approached Lynch and initiated the unplanned chat.

Speaking on Friday at Aspen Ideas Festival in Aspen, Colorado, Lynch indicated her regrets about not having turned down the former president.

She also announced that she "fully" expected herself to accept recommendation by FBI investigators on whether to bring charges related to Hillary Clinton's personal email setup.

Lynch said her decision to accept FBI recommendation was made long before her impromptu chat with Mr. Clinton.

In March 2015, Clinton acknowledged that she had exchanged about 60,000 emails from her private email account during her stint in the Obama administration, among which about half were personal and thus deleted.

All emails were sent and received via a private email server based at Clinton's home.

In response to requests from the State Department, the Clinton camp turned over the other half, roughly 30,000 emails in total, to the State Department in December 2014.