WASHINGTON >> On the morning of March 13, 2011, the assistant secretary of state for Near Eastern affairs, Jeffrey D. Feltman, wrote an urgent email to more than two dozen colleagues informing them that Saudi Arabia and the United Arab Emirates were sending troops into Bahrain to put down anti-government protests there.

Feltman’s email prompted a string of 10 replies and forwards over the next 24 hours, including to Secretary of State Hillary Clinton, as the Obama administration debated what was happening and how to respond.

The chain contained information now declared classified, including portions of messages written by Feltman; the former ambassador in Kuwait, Deborah K. Jones; and the current director of the CIA, John O. Brennan.

The top administration officials discussed the Bahrain situation on unclassified government computer networks, except for Clinton, who used a private email server while serving as secretary of state.

Her server is now the subject of an FBI investigation, which is likely to conclude in the next month, about whether classified information was mishandled.

Whatever the disposition of the investigation, the discussion of troops to Bahrain reveals how routinely sensitive information is emailed on unclassified government servers, reflecting what many officials describe as diplomacy in the age of the Internet, especially in urgent, fast-developing situations.

A review of the 30,322 emails from Clinton’s private server that the State Department has made public under the Freedom of Information Act provide an extensive record of how such sensitive information often looped throughout President Barack Obama’s foreign policy apparatus on unclassified systems, from embassies to the United Nations to the White House.

The senders included Denis R. McDonough, currently the White House chief of staff and previously the deputy national security adviser, and Susan E. Rice, the former U.S. representative at the United Nations who is now Obama’s national security adviser.

Many of the emails were sent over the State Department’s unclassified system, state.gov, which is considered secure but not at the level of the State Department’s system for emailing classified information.

At the State Department, the Pentagon and the White House, among other agencies, officials have two systems for email, one for classified messages and one for more routine business. They are nicknamed the “high side” and the “low side.”

Clinton’s private server — set up in her home in Westchester County, N.Y. — was assumed to be even less secure than the State Department’s “low side,” although the unclassified servers at some government agencies have been hacked in recent years.

One result of Clinton’s decision to maintain a private server is that it has put State Department officials on the defensive about their use of state.gov for some business that might be considered classified.

Of the 30,322 emails made public, 2,028 have had portions redacted and are now classified at the lowest level of classification, “confidential.”

Nearly three-quarters of those emails were classified because they contained what is called “foreign government information” — a vast category of information, gathered through conversations and meetings with foreign counterparts that are the fundamentals of diplomacy, but which had to be protected when the emails were released.

Last week, in an apparent attempt to dispel criticism that many of the emails were improperly sent, a top State Department official argued in a letter to three Senate Democrats that the nation’s diplomats and officials were in fact allowed to send “foreign government information” through the government’s unclassified computer systems.

“Department officials of necessity routinely receive such information through unclassified channels,” said the letter, dated May 2 and written by the assistant secretary of state for legislative affairs, Julia Frifield.

“For example, diplomats engage in meetings with counterparts in open settings, have phone calls with foreign contacts over unsecure lines, and email with and about foreign counterparts via unclassified systems.”

The letter went on to say that using “foreign government information” in unclassified emails “does not amount to mishandling the information.”

The State Department, unlike some other federal agencies, does not have the authority to redact that category of information even if it is required to release documents under the Freedom of Information Act.

Thus, the only way the State Department could withhold “foreign government information” in the emails being released under court order was to classify it, according to the letter.

The letter was a reply to one sent in March to Sens. Patrick J. Leahy of Vermont, Thomas R. Carper of Delaware and Dianne Feinstein of California. A copy was given to The New York Times by a government official who believed the classification of the emails was unfairly implicating diplomats and other officials conducting diplomacy in the modern era.

Of the 30,322 emails, the FBI’s investigation has focused on a smaller number, including 22 that the CIA insisted contained information classified “top secret.”

Those emails have not been released, even with redactions, because they include material classified at the highest levels, known as “top secret/SAP,” according to a letter from the inspector general of the nation’s intelligence agencies, I. Charles McCullough III.

That designation refers to “special access programs,” which are among the nation’s most guarded secrets. The emails are said to include references to, among other things, the CIA’s program to hunt and kill suspected terrorists with armed drones in Pakistan.

An additional 65 emails, which have been released, have had portions redacted because they included information classified at the level of “secret.”

One exchange of emails typical of those now classified because they contain “foreign government information” involved McDonough, Rice and her deputy at the time, Rosemary A. DiCarlo, and the Palestinian effort in September 2011 to be recognized as a state by the United Nations.

The exchange included eight separate emails, all sent on unclassified networks. Of those, six were redacted almost completely when the State Department released them in January.

According to the subject line and what information does appear, the three discussed deliberations between U.N. Secretary-General Ban Ki-moon and the Palestinian president, Mahmoud Abbas, as well as Rice’s discussion with the Palestinian representative at the United Nations, Riyad H. Mansour, on the bid for statehood — all instances of “foreign government information.”

The chain of emails eventually encompassed 16 officials, including political appointees and career diplomats, and was ultimately forwarded to Clinton’s inbox by Jake Sullivan, her deputy then and now the senior policy adviser for her election campaign.

Philip H. Gordon, an assistant secretary of state under Clinton, said, “If all these respected, senior foreign service officers and experienced ambassadors are sending these emails, then this issue is not about how Hillary Clinton managed her email, but how the State Department communicates in the 21st century.”

Gordon, later a special assistant to the president for the Middle East, wrote more than 40 emails that were redacted on the grounds that they contained classified information.

Clinton herself wrote, responded to or forwarded 96 emails that have been classified in part, including one that is classified secret; 46 of those contained the “foreign government information” that the department’s letter addressed.

There are, to be sure, other emails that do not fall into the category of “foreign government information,” and some raise questions about the sort of information senior officials sent in unclassified emails.

In 18 emails, for example, information has been classified on the grounds that it identifies CIA officials, including two instances that are now considered “secret.”

One of those was a seemingly benign photo opportunity listed on Clinton’s daily schedule, with the person who gave her a daily intelligence briefing, making it obvious that the person was an agency employee.

That email was originally released as “confidential” but upgraded to “secret,” probably reflecting that the person holds an undercover position now.

Another exchange involving the CIA came the day after David H. Petraeus resigned as the agency’s director in November 2012. Brennan, then still at the White House, sent an email — detailing the provisions for Petraeus’ personal security following his surprise resignation — to Thomas E. Donilon, Obama’s national security adviser. Donilon then forwarded it to Clinton.

“Madam Secretary — Attached is an update on the security for Dave P.,” he wrote. The entirety of Brennan’s note has now been redacted and classified as “confidential” on the grounds that it involves “vulnerabilities or capabilities of systems, installations, infrastructures, projects, plans, or protection services relating to the national security.”

Petraeus ultimately pleaded guilty to a misdemeanor for keeping highly classified information in eight black notebooks he kept in his home, including such details as the names of covert officers and programs, and sharing them with his biographer and lover.

A spokesman at the CIA, Ryan Trapani, said in a statement that Brennan had believed that the information he sent in the email was unclassified.

“When operating in a position like he was at the White House, officials often have to make spot judgments about whether information is classified or not,” he wrote.

“In most cases, the determinations are correct, but in some situations, another agency may consider certain information classified that the author does not.”

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