This post has been updated.
Clinton turned 30,000 e-mails over to the State Department in December 2014, and the department now has a team reviewing the correspondence to determine what should be released and what should be redacted under laws that allow the government to withhold public documents from release on a variety of grounds, including national security.
A federal judge has ordered the State Department to release the cleared documents to the public on a rolling basis, with all of them to be available by January.
The team, which had included only State Department personnel, was expanded to include reviewers from five intelligence agencies after an e-mail was released in May that intelligence personnel said contained classified material that should not have been made public.
In the status update to the court, an attorney for the State Department indicated that the reviewers from the intelligence community have identified 305 e-mails out of a sample of 20 percent of Clinton's e-mails to send for further review by their agencies. Those agencies may not ultimately conclude that the e-mails contain classified information.
But the mounting number of possibly sensitive e-mails illustrates the way in which the issue is expanding as a political headache for Clinton's presidential campaign.
"What you're seeing here is exactly what we want to see, which is the proper care and scrutiny being applied to this," State Department spokesman John Kirby told reporters Monday. "It's a healthy thing. It's a good thing. It doesn't mean that all 300 are going to end up at some level of upgrade. I suspect some will, and I suspect some won't."
When her use of a private server was first made public in March, Clinton said she did not use her e-mail account to exchange classified information. However, the Inspector General for the Intelligence Community has said he located two e-mails that contained information that was top secret, the highest level of classification, in a sample of 40 e-mails he reviewed. The State Department has said Clinton did not write the e-mails in question, and it is reviewing the IG's finding.
Clinton's campaign has more recently said none of her e-mails were marked classified. Experts say it is the responsibility of an e-mail's author to handle classified material correctly, including by using proper markings to indicate its presence.
The FBI is now investigating Clinton's e-mail set-up, as are members of Congress. At a political dinner on Friday, Clinton chalked the investigations up to partisan politics. "It's not about e-mails or servers," she said. "It's about politics."
Nick Merrill, a spokesman for Clinton's campaign, said the additional intelligence community review was "not surprising given the sheer volume of intelligence community lawyers now involved in the review of these emails."
"We expect there will continue to be competing assessments among the various agencies about what should and shouldn't be redacted," he said in an e-mail. "It remains our hope that this process not get hampered by bureaucratic considerations and that her emails be released as quickly and transparently possible."