(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.

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.

"It's certainly possible that for any number of reasons, traffic can be sent that's not marked appropriately for its classification. That is certainly possible," Kirby said.

But he added that he wasn't going to make any judgments about this particular case.

"All I can tell you definitively is it wasn't marked classified at the time it was sent," Kirby said.

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."

'Over-classification run amok'

Clinton did not mention the email controversy during campaign events Friday. But in an interview with KCRG in Dubuque, Iowa , Clinton maintained that she "never emailed anything that was considered to be classified" and asked for the emails to be released.

"I'd love for people to see what I did and I hope that will happen," she said.

Brian Fallon, a spokesman for Clinton's campaign, said in a statement that Friday's announcement was a case of "over-classification run amok" and reiterated Clinton's position that the emails be made public.

But later Friday, Fallon declined to say whether Clinton would ask Obama to declassify the emails when pressed by CNN's Wolf Blitzer on "The Situation Room."

"The President easily could declassify all of these emails if she asked him and if he agreed, right?" Blitzer asked.

"I'd really be surprised if this has risen to the President's level," Fallon replied. "Because, again, this a mundane matter of fulfilling a FOIA request."

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."

More emails released

The State Department released more than 900 of Clinton's emails Friday -- 242 of which received classification upgrades: 11 to "secret" and 209 more to "confidential," along with the 22 emails containing "top secret" information -- but the release fell well behind the judge-imposed timetable for producing all of her emails.

Among the most interesting correspondence:

Clinton seeking to confirm with her team that a piece of information, which top aide Jake Sullivan described as proof the new Center for Strategic Counterterrorism Communications was doing its job, was "not classified or otherwise inappropriate" and fit to share with a reporter;

Evidence that Clinton actively lobbied on behalf of the Affordable Care Act -- at one point, she emailed Miguel Rodriguez, then the director of the White House Office of Legislative Affairs, to inform him she'd spoken and received a "yes" from then-Arkansas Rep. Marion Berry.

John Podesta, now the chairman of Clinton's campaign, emailing aide Cheryl Mills to ask if she had a personal email to which he could send a message hours ahead of a meeting with then-chief of staff Rahm Emanuel.

Clinton asking, "So, can someone sign just my first name?" when presented with a farewell letter (written by Huma Abedin) to former Sen. George Mitchell as he prepared to leave after two years as the top U.S. envoy to the Israelis and Palestinians.

The emails have been publishing over the last eight months more or less in accordance with a schedule 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.

Republicans pounce

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

"The new e-mail release is a disaster for Hillary Clinton. At a minimum, how can someone with such bad (judgment) be our next president?" GOP front-runner Donald Trump tweeted.

The new e-mail release is a disaster for Hillary Clinton. At a minimum, how can someone with such bad judgement be our next president? — Donald J. Trump (@realDonaldTrump) January 29, 2016

Florida Sen. Marco Rubio said Clinton's email use was a "disqualifier" for the White House.

"Hillary Clinton put some of the highest, most sensitive intelligence information on her private server because maybe she thinks she's above the law," Rubio said at a town hall event in Clinton, Iowa.

Texas Sen. Ted Cruz told conservative radio host Hugh Hewitt that Clinton's email controversy would seriously imperil her presidential aspirations.

"We are talking about serious offenses for which the Obama Justice Department threw the book at General (David) Petraeus," Cruz said. "And justice needs to be enforced fairly and impartially."

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."

With even more "top-secret" emails on her secret server, Clinton has removed all doubt that she cannot be trusted with the presidency — Reince Priebus (@Reince) January 29, 2016

But Rep. Adam Schiff, D-California, the ranking Democrat on the House Intelligence Committee, said classification determinations "are often very complex."

"It's important to remember that none of these emails had any classification markings at the time they were sent, and Secretary Clinton and her staff were responding to world events in real time without the benefit of months of analysis after the fact," Schiff said.

Meanwhile, Clinton's top Democratic 2016 rival, Vermont Sen. Bernie Sanders, said in a statement that "there is a legal process in place which should proceed and not be politicized."

"The voters of Iowa and this nation deserve a serious discussion of the issues facing them," Sanders said, and referenced the Democratic debate in October , when he memorably declined to attack Clinton over the issue.

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