US spy agencies have told the US Congress that Hillary Clinton's home computer server contained some emails that should have been treated as top secret because their wording matched sections of some of the government's most highly classified documents, four sources familiar with the agency reports said.
The two reports are the first formal declarations by US spy agencies detailing how they believe Clinton violated government rules when highly classified information in at least 22 email messages passed through her unsecured home server.
The State Department has already acknowledged that the emails contained top secret intelligence, though it says they were not marked that way. It has not previously been clear if the emails contained full classified documents or only some information from them.
The agencies did not find any top secret documents that passed through Clinton's server in their full version, the sources from congress and the government's executive branch said.
However, the agency reports found some emails included passages that closely tracked or mirrored communications marked "top secret", according to the sources, who all requested anonymity. In some cases, additional classification markings meant access was supposed to be limited to small groups of specially cleared officials.
Under the law and government rules, US officials and contractors may not transmit any classified information - not only documents - outside secure, government-controlled channels.
Such information should not be sent even through the government's .gov email network.
The front runner for the Democratic nomination for president and former secretary of state has insisted she broke no rules.
Clinton's lawyer, David Kendall, did not respond to a request for comment. Clinton campaign spokespeople did not respond to multiple requests for comment.
Two sources said some of the top secret material was related to the CIA's campaign of drone strikes against Islamist militants in the Middle East and South Asia.
That campaign has been widely reported by Reuters and other media outlets, but it officially is classified as a "Top Secret/Special Access Program", meaning only a limited number of people whose names are on a special list are allowed to learn details about it.
One source said the reports identified some information in messages on Clinton's server that came from human sources, such as confidential CIA informants, and some from technical systems, such as spy satellites or electronic eavesdropping.
The Clinton campaign criticised the State Department's decision last month to withhold the 22 emails containing top secret information from the public, blaming it on "bureaucratic infighting" and "over-classification run amok".
"As we have previously made clear, we are not going to speak to the content of the emails," a State Department official said on Wednesday when asked about the intelligence agency reports.
Two of the sources told Reuters that one of the reports on the emails came from the CIA. Three sources said the other report came from the National Geospatial Intelligence Agency, which analyses US spy satellite intelligence.