Former secretary of state Hillary Clinton has some 'splaining to do, after a damaging email surfaced early Friday that appeared to show her instructing an aide to send classified information through 'nonsecure' email - after removing classified headings from the document.

The 2011 email was included in more than 3,000 pages of emails released by the State Department at 1 a.m. Friday, as part of an effort to make up for a late December release that was smaller than expected.

The emails - which have been ordered by a federal judge - have dogged Clinton's front-running presidential campaign for almost the past year.

Republicans hammered Clinton over both the timing and the content of Friday's release.

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Former Secretary of State Hillary Clinton, shown here testifying on Capitol Hill in October, has come under fire for a 2011 email that critics say proves she circumvented federal law

This 2011 email was included in a batch of more than 3,000 pages of Hillary Clinton emails that were released at 1 a.m. Friday

Clinton has testified twice before the House Select Committee on Benghazi, most recently in October

The House Select Committee on Benghazi released this statement later Friday to clarify that it is not investigating Clinton's potential mishandling of classified data through her private, home-based email server - and going on to claim credit for the discovery of her email setup

The bombshell 2011 email concerned a classified set of talking points that Clinton's staff was trying to send over fax.

'They say they've had issues sending secure fax. They're working on it,' aide Jacob Sullivan wrote Clinton after it was discovered the document hadn't been sent to Clinton the previous evening as expected.

'If they can't, turn into nonpaper w no identifying heading and send nonsecure,' Clinton replied.

The email exchange is partially redacted, making it difficult to ascertain more specifics about the episode, but it appears to be a potential troublemaker for the Clinton campaign.

A Republican-controlled House committee last March discovered that Clinton had used a private, home-based email server while she was secretary of state from 2009 to 2013, creating the controversy that still lingers, since the FBI is now investigating the potential mishandling of classified information.

'If they can't, turn into nonpaper w no identifying heading and send nonsecure,' - Email from Hillary Clinton, June 17, 2011

Sen. Chuck Grassley, an Iowa Republican who chairs the Senate Judiciary Committee, said in a statement Friday that the 2011 email was 'disturbing.'

Grassley noted that the email release came in response to a request under the federal Freedom of Information Act, over which his committee has jurisdiction.

'The State Department’s latest Freedom of Information Act release contains a disturbing email that appears to show the former Secretary of State instructing a subordinate to remove the headings from a classified document and send it to her in an unsecure manner.'

'It raises a host of serious questions and underscores the importance of the various inquiries into the transmittal of classified information through her non-government email server. How long has the State Department been aware of this email? Why is it just now being released? Was her instruction actually carried out? If so, has the FBI opened a criminal inquiry into these circumstances?'

Clinton came under swift fire from other Republican critics Friday, especially the Republican National Committee.

'This appears to be part of a pattern of behavior by Hillary Clinton where protocols designed to protect national security and our diplomatic efforts were cast aside,' RNC spokesman Michael Short said.

'Serious legal questions are raised by these emails given the fact Clinton signed non-disclosure agreements obligating her to protect classified information regardless of whether it was marked.'

Clinton at her October testimony before the House panel

House Select Committee on Benghazi Chairman Trey Gowdy, a South Carolina congressman, has been accused of playing politics with the panel to damage Clinton's presidential prospects

Sen. Tom Cotton, from Clinton's adopted home state of Arkansas, also blistered her with his own statement.

'Once again it’s clear Hillary Clinton prioritizes her personal convenience over national security. It’s as if she thinks the rules just don’t apply to her and is hypocrisy of the highest order,' Cotton said.

'Any other federal employee found acting in such a manner would likely have their clearance revoked, employment terminated, and could face prosecution. Instead, Hillary Clinton is in the running for a major promotion.'

The Clinton campaign has refused to answer media requests for comment about the 2011 email throughout Friday.

At the White House, Press Secretary Josh Earnest dodged a question about the email but later appeared to defend Clinton's handling of the information.

'It is not uncommon for the administration, in pursuit of transparency, to release redacted information that if un-redacted were to otherwise would be subject to some classification,' he said.

'Again, without knowing the details of what exactly was being discussed it’s hard for me to comment.'

On Capitol Hill, the House Select Committee on Benghazi issued a statement clarifying that the FBI, not the House panel, that is investigating the potential mishandling of classified information.

The statement went on to suggest that the committee deserves credit for uncovering Clinton's email setup last year.

'Of course, none of the Secretary of State's emails – including one in which she appears to instruct a top aide to strip a document of its 'identifying heading and send nonsecure' instead of via classified, secure fax – would have been discovered if not for the work of the Select Committee on Benghazi.'

At the State Department late Friday afternoon, a spokesman would not comment on the 2011 email and said he had no other information about it anyway, including whether or not it has been forwarded to the Justice Department for inclusion in the FBI inquiry.

Spokesman John Kirby refused to comment any further.

'I'm going to continue to refrain from speaking to specific content here. That's not our role right now,' Kirby said according to a report in The Washington Examiner.

'I'm not going to talk about former Secretary Clinton's email practices. Those practices are under review and investigation so it wouldn't be appropriate for me to talk about that.'