President Barack Obama used a fake name while communicating with Hillary Clinton while she used a private email server during her time as US secretary of state, FBI documents released Friday revealed.

The documents offer some details about the agency's investigation into Clinton's use of a private email server, a matter that has continually plagued the Democratic presidential nominee's campaign.

According to the documents, Huma Abedin, one of Clinton's aides, first noticed the former secretary of state's exchange with an unrecognized sender on June 28, 2012 that was later revealed to be Obama.

"Once informed that the sender's name is believed to be pseudonym used by the president, Abedin exclaimed: 'How is this not classified?'" the report says.

The State Department has refused to released the emails between Obama and Clinton, according to Politico.

Over the course of the investigation, the FBI conducted interviews with members of Clinton's staff and also former Secretary of State Colin Powell and former CIA acting director Mike Morell, among others.

Clinton has maintained that she did not knowingly handle any classified information while she used a private email server.

According to the FBI documents, an IT employee managing Clinton’s server initiated a policy of deleting any emails older than 60 days.

Clinton's Republican presidential rival, Donald Trump has cited the email server issue to label Clinton as untrustworthy.

A Trump campaign spokesperson fired off a response to the latest findings Friday night:

“The fact an IT staffer maintaining Clinton’s secret server called a new retention policy designed to delete emails after 60 days a 'Hillary coverup operation' suggests there was a concerted effort to systematically destroy potentially incriminating information," said Jason Miler, senior communications advisor for the Trump campaign.

"It’s no wonder that at least five individuals tied to the email scandal, including Clinton’s top State Department aide and attorney Cheryl Mills, secured immunity deals from the Obama Justice Department to avoid prosecution," he added.

The FBI said in July that it would not recommend charges against Hillary Clinton in the matter.