WASHINGTON (AP) — During the 2012 attack on U.S. facilities in Benghazi, Libya, State Department officials in Washington were emailing one another with updates in real time. Embedded in those messages were nuggets of classified information, including an apparent reference to a CIA facility that was a closely guarded secret.

When the emails were released last year under the Freedom of Information Act, that and other information found to be classified were censored from public view.

Hillary Rodham Clinton is now under fire after revelations that material deemed classified was sent through the unsecure home server she was using as secretary of state for both official and private email, a matter that has become an issue in her front-running campaign for the Democratic presidential nomination.

Records reviewed by The Associated Press make clear that the leakage of secrets onto unsecure email is a State Department problem dating to the George W. Bush administration, if not earlier. It may have accelerated under Clinton as email use became more frequent.

Clinton's use of a home server makes her case unique, but it's not clear whether the security breach would have been any less had she used government email. The State Department only systematically checks email for sensitive or classified material in response to a public records request.

Clinton insists she didn't send or receive information marked classified, and there is no evidence she did. But information can be classified without being marked. Government investigators have found material they believe was classified when sent in several dozen of 30,000 emails that the former secretary of state has turned over.

In five messages that date to Condoleezza Rice's tenure as secretary of state during the George W. Bush administration, large chunks are censored on the grounds that they contain classified national security or foreign government information.

In a December 2006 email, for example, diplomat John J. Hillmeyer appears to have pasted the text of a confidential cable from Beijing about China's dealings with Iran and other sensitive matters. Large portions of the email were marked classified and censored before release.

Released under the Freedom of Information Act, the emails have been posted on the State Department's searchable reading room.

On the night of the Benghazi attack on Sept. 11, 2012, department officials emailed one another about extraordinarily sensitive matters, including the movement of Libyan militias and the locations of key Americans.

An email from diplomat Alyce Abdalla, sent the night of the attack, appears to report that the CIA annex in Benghazi was under fire. The email has been largely whited out, with the government citing the legal exemption for classified intelligence information. The existence of that facility is now known; it was a secret at the time.

In an email sent at 8:51 p.m. on Sept. 11, 2012, Eric J. Pelofsky, a senior adviser to then-U.N. Ambassador Susan Rice, gives an update on efforts to locate U.S. Ambassador Chris Stevens, who died in the attack.

The email was marked unclassified when sent. Later, part of it was deemed classified and censored before its release.

Critics have said Clinton and her aides should have known not to discuss anything remotely secret over unsecured email. The emails show they were cognizant of security, routinely communicating over secure phone and fax lines.

Many of the emails to Clinton containing classified information were forwarded to her by a close aide, Huma Abedin. Most, however, originated with diplomats who have access to confidential material. Some emails sent by Clinton have since been censored.

Such slippage of classified information into regular email is "very common, actually," said Leslie McAdoo, a lawyer who frequently represents government officials and contractors in disputes over security clearances and classified information.

What makes Clinton's case different is that she exclusively sent and received emails through a home server in lieu of the State Department's unclassified email system. Neither would have been secure from hackers or foreign intelligence agencies, so it would be equally problematic whether classified information was carried over the government system or a private server, experts say.

In fact, the State Department's unclassified email system has been penetrated by hackers believed linked to Russian intelligence.

Many of the emails to Clinton came from state.gov email accounts, noted Steven Aftergood, an expert on classification at the Federation of American Scientists. "So if there is routine security screening and monitoring of incoming and outgoing State Department emails, anything that is classified should have been flagged. That does not seem to have happened. I think it's the State Department culture."

That may be true, but it would not save a rank-and-file official with a security clearance who was caught sending classified information over email, said Bradley Moss, a lawyer who frequently represents intelligence officers. That person could lose his job, his clearance, or both.

"In real life, the 'everybody does it defense' doesn't fly," Moss said.

Clinton, speaking to reporters after an event in Iowa, said: "My use of personal email was allowed by the State Department. It clearly wasn't the best choice. I should have used two emails, one personal, one for work. And I take responsibility for that decision."

The AP has asked the State Department to turn over records reflecting any concerns by agency computer staff or security officials over Clinton's use of a private email server, but has received no responsive documents.

Clinton also had access to a classified messaging system, but it's not widely used at the State Department. Most department officials in Washington and at embassies have on their desktops a classified network that goes up to "secret" level. A small number of State officials, including the secretary, can use a third system that goes up to "top secret" level in special secure rooms.

But even the middle-tier "secret" network is cumbersome for many in the agency, said officials who would not be quoted when discussing internal security policies. Only a few top officials in Washington are able to read classified emails outside the department's headquarters. Most ambassadors can't open their accounts from home. Officials in the field may have no access at all.

Lots of State Department information is meant for use, sharing and interaction with foreign officials, the vast majority of whom aren't authorized to receive classified U.S. material.

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Associated Press writers Steven Braun, Bradley Klapper, Deb Riechmann and Jack Gillum contributed to this report.

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