By Laura Koran, CNN–

(CNN)The State Department announced Friday that it will not release 22 emails from former Secretary of State Hillary Clinton because they contain “top secret” information, the highest level of government classification.

The decision, coming three days before the Iowa caucuses, could provide fodder for Clinton’s political opponents, especially Republicans, who are likely to make note of the emails’ “top secret” designation. Clinton’s email use has haunted her on the campaign trail since it became public early last year that she maintained a private server while leading the State Department.

State Department spokesman John Kirby said the documents, totaling 37 pages, were not marked classified at the time they were sent, but are being upgraded at the request of the Intelligence Community because they contain sensitive information.

“We are aware that there is intense interest in this matter, and we are announcing this decision now because the (Freedom of Information Act) process regarding these emails has been completed,” Kirby said. “While we have requested a month’s extension to complete the entire review, we did not need the extension for these documents.”

But, Kirby said, a separate review by the bureaus of Diplomatic Security and Intelligence and Research is being held into whether the information in the emails was classified at the time they were sent and received. He would not say when the review began or how long it would go, and acknowledged it’s possible there could be classified emails that weren’t marked as such.

A senior State Department official said the review “began very recently” and was initiated by the State Department, but the official wouldn’t say what prompted it.

A spokesperson for the Intelligence Community’s inspector general declined to comment.

Kirby also said 18 emails, comprised of eight email chains between Clinton and President Barack Obama, are being “withheld in full” to “protect the President’s ability to receive unvarnished advice and counsel.” But, Kirby said, they “have not been determined to be classified” and said they will “ultimately be released in accordance with the Presidential records act.”

Brian Fallon, a spokesman for Clinton’s campaign, said in a statement that Friday’s development was a case of “over-classification run amok.”

“We firmly oppose the complete blocking of the release of these emails. Since first providing her emails to the State Department more than one year ago, Hillary Clinton has urged that they be made available to the public. We feel no differently today,” Fallon said.

Fallon also contended on MSNBC that the decision to withhold the 22 emails is “happening at the behest of other agencies in the government who have hijacked the process that’s been taking place for the last several months.”

Asked Friday if he had “certainty and confidence” that Clinton will not be indicted over the email controversy, White House Press Secretary Josh Earnest said any decision to prosecute Clinton would rest with the Justice Department.

“That is a decision to be made solely by independent prosecutors,” Earnest said. “But again, based on what we know from the Department of Justice, it does not seem to be headed in that direction.”

Republicans pounce

Several prominent Republicans, including presidential hopefuls, quickly condemned Clinton, the Democratic 2016 front-runner, over Friday’s developments.

“Hillary Clinton put some of the highest, most sensitive intelligence information on her private server because maybe she thinks she’s above the law,” Florida Sen. Marco Rubio said at a town hall event in Clinton, Iowa. “Or maybe she just wanted the convenience of being able to read this stuff on her Blackberry. This is unacceptable. This is a disqualifier.”

Retired neurosurgeon Ben Carson said in a Facebook post that he “won’t keep secrets from the American people.”

“This election is about who will defeat Hillary Clinton — I am best suited to do that because I have the best interests of We the People in mind,” he said.

And Republican National Committee Chairman Reince Priebus tweeted that Clinton and the Obama administration have “obfuscated and misled at every available opportunity,” adding that she has “removed all doubt that she cannot be trusted with the presidency.”

Campaign officials for Vermont Sen. Bernie Sanders, Clinton’s top competitor for the Democratic nomination, did not immediately respond to requests for comment. But at a Democratic debate in October, Sanders memorably declined to attack Clinton over the issue.

“The American people are sick and tired of hearing about your damn emails,” he said to applause.

More emails to be released

The State Department will release another batch of Clinton’s emails Friday, but the release is expected to fall well behind the judge-imposed timetable for producing all of her emails.

The emails have been publishing over the last eight months more or less in accordance with aschedule set by Judge Rudolph Contreras, with increasingly large batches uploaded to a State Department website at the end of each month.

This month’s release was supposed to be the final one and include just over 9,000 pages of documents — the largest number to date.

But last Thursday, the State Department filed a motion to extend the final productions until February 29 because the department had failed to send more than 7,000 pages of those emails to other government agencies for review, only recognizing the mistake earlier this month.

That delay was then compounded by a huge snowstorm that shut down the federal government for several days, according to the State Department’s motion.

In a separate filing Thursday night, lawyers for the State Department said the State Department “candidly acknowledged — and regrets — that it was responsible for the failure to send the documents for consultation and that it was simply a mistake that occurred during the enormous undertaking of reviewing and processing the entire Clinton email collection in a compressed time frame.”

A State Department official told CNN Thursday, “State Department staff are working extremely hard to get as many emails are through our FOIA process as possible,” but wouldn’t elaborate on what was in the legal filing.

Contreras has not yet ruled on the State Department’s request for more time. But regardless of his ruling, the State Department is unlikely to meet its full production quota since, as it acknowledges in Thursday’s filing, some of the emails flagged for further review had not even been sent to a dozen relevant agencies for review.

“State has experienced some difficulty contacting some of the appropriate agency personnel since the snow storm and is still making arrangements with some of the receiving agencies for secure delivery of the documents,” the department lawyers wrote, emphasizing that these represent a small portion of the total remaining emails.

Lawyers for the plaintiff in the Freedom of Information Act case have submitted their own filing opposing the State Department’s request for more time.

The delay, they note, pushes the final release back until after the early presidential primaries, causing “grave, incurable harm.”

In May, Contreras ordered the State Department to “aspire to abide” by the monthly production schedule. And while the timeline he set is aspirational, the department must also submit reports each month to explain its progress.