WASHINGTON — They risk imprisonment or death stealing the secrets of their own governments. Their identities are among the most closely protected information inside American intelligence and law enforcement agencies. Losing even one of them can set back American foreign intelligence operations for years.
Clandestine human sources are the lifeblood of any espionage service. This helps explain the grave concern within American agencies that information from undercover sources was included in some of the classified documents recently removed from Mar-a-Lago, the Florida home of former President Donald J. Trump — raising the prospect that the sources could be identified if the documents got into the wrong hands.
Mr. Trump has a long history of treating classified information with a sloppiness few other presidents have exhibited. And the former president’s cavalier treatment of the nation’s secrets was on display in the affidavit underlying the warrant for the Mar-a-Lago search. The affidavit, released in redacted form on Friday, described classified documents being found in multiple locations around the Florida residence, a private club where both members and their guests mingle with the former president and his coterie of aides.
Nothing in the documents released on Friday described the precise content of the classified documents or what risk their disclosure might carry for national security, but the court papers did outline the kinds of intelligence found in the secret material, including foreign surveillance collected under court orders, electronic eavesdropping on communications and information from human sources — spies.
Mr. Trump and his defenders have claimed he declassified the material he took to Mar-a-Lago. But documents retrieved from him in January included some marked “HCS,” for Human Intelligence Control System. Such documents have material that could possibly identify C.I.A. informants, meaning a general, sweeping declassification of them would have been, at best, misguided.
“HCS information is tightly controlled because disclosure could jeopardize the life of the human source,” said John B. Bellinger III, a former legal adviser to the National Security Council in the George W. Bush administration. “It would be reckless to declassify an HCS document without checking with the agency that collected the information to ensure that there would be no damage if the information were disclosed.”
C.I.A. espionage operations inside numerous hostile countries have been compromised in recent years when the governments of those countries have arrested, jailed and even killed the agency’s sources.
Last year, a top-secret memo sent to every C.I.A. station around the world warned about troubling numbers of informants being captured or killed, a stark reminder of how important human source networks are to the basic functions of the spy agency.
During the early part of last decade, the Chinese government dismantled the C.I.A.’s network of sources within China — crippling the agency’s spying operations in the country for years. Source networks in Iran and Pakistan have also been compromised, prompting the agency to ask its case officers and analysts to redouble the efforts to protect the identities of spies and informants.
Even a single source, if well placed, can be of amazing importance to the spy agency. When one informant, critical to the intelligence assessment that President Vladimir V. Putin of Russia favored the election of Mr. Trump, had to be extracted and resettled in Virginia, the C.I.A. was, for a time, left somewhat in the dark about senior levels of Kremlin decision-making.
In 2010, when WikiLeaks and several news outlets, including The New York Times, published thousands of American diplomatic cables from State Department employees posted around the world, the greatest concern among American officials was the possibility that foreign sources aiding the United States might be identified by name in the documents.
When F.B.I. agents in May went through the 15 boxes of material turned over to the National Archives by Mr. Trump in January, a year after he left office, they quickly determined that they contained 184 documents marked as classified, including some labeled HCS — an especially troublesome revelation in the eyes of intelligence experts.
“It is among the most sensitive information relating to human intelligence sources and very tightly held at the C.I.A.,” said George Jameson, a former senior C.I.A. officer and lawyer. “A compromise could result in harm to the source and the source’s information.”
An intelligence document marked HCS will contain details about the source of the information. Often such descriptions are very general, noting if a “clandestine source” has direct or secondary knowledge of the intelligence presented. But sometimes there are more direct descriptions to help policymakers properly assess the information, details that could allow people reading the document to identify the source — a prime reason the spy agency seeks to tightly control HCS documents.
The HCS designation is “used to protect exceptionally fragile and unique” human intelligence operations and methods “that are not intended for dissemination outside of the originating agency,” according to a 2013 directive from the Office of the Director of National Intelligence.
According to former officials, documents marked HCS have special handling requirements to make sure they are stored properly and not reviewed by people who are not cleared to see them.
“Although the president generally received finished intelligence that included HCS reporting, this would include source descriptions and context to establish the information’s reliability, details that would enable an adversary to narrow down from whom, and where, the secrets came,” said Douglas London, a former C.I.A. officer who was a top counterterrorism official during the Trump administration. “The more sensitive the information, the fewer the suspects or technical vulnerabilities for the adversary to investigate.”
In addition to the HCS markings, some of the documents were marked FISA, indicating information collected under the Foreign Intelligence Surveillance Act.
“What this tells us is that there was possibly something from human beings, from spies, possibly something involving foreigners who are the only ones targeted under FISA and potentially there is very sophisticated sensitive information involved here,” said Glenn S.Gerstell, the former general counsel of the National Security Agency.
Ultimately, Mr. Gerstell said, understanding how sensitive any of the documents are, and what sources might be compromised, requires the documents to be examined by the Office of the Director of National Intelligence. Such an examination is one reason the Justice Department and the F.B.I. conducted the search at Mar-a-Lago to collect the material.
“One of the reasons they need to get these documents is to understand what is in there for the purpose of conducting a damage assessment,” Mr. Gerstell said. “We have surveillance tapes and we will see who had access. But the government also needs to see the documents so they can know what might have been compromised.”
The House and Senate Intelligence Committees have requested such a review, but it is not clear when the intelligence community will begin such an examination. On Friday, Senator Mark Warner, the Virginia Democrat who leads the Senate Intelligence Committee, reiterated his call for an assessment of the damage the mishandling of the documents may have caused.
“It appears, based on the affidavit unsealed this morning, that among the improperly handled documents at Mar-a-Lago were some of our most sensitive intelligence,” Mr. Warner said.
Until more about the nature of the documents is publicly known it is impossible to tell what, if any damage was done. But former officials stressed that counterintelligence experts often will take measures to protect sources or change collection methods if they believe a classified document could have been viewed by people not authorized to see it.
“It is a principle of counterintelligence that when you believe a code or classified material has been possibly compromised you have to assume the worst,” Mr. Gerstell said. “It is a powerful reason to know what is in the documents and who had access.”
Adam Goldman contributed reporting.