The FBI told a federal court it needed a search warrant to look at thousands of Hillary Clinton's emails on the laptop of former US congressman Anthony Weiner because they had potential to cause "grave damage to national security" if disclosed.
The wording was contained in a redacted search warrant and other court papers that were previously under seal until today in the investigation of an online relationship between Mr Weiner, the estranged husband of top Clinton aide Huma Abedin, and a teenage girl in North Carolina.
The papers offered no new revelations about Mrs Clinton's emails or the scope of a case that factored into the presidential election.
A sworn statement dated October 30 from an FBI agent whose name was blacked out cited a previous inquiry into Mrs Clinton's personal computer server that found 22 emails classified "Top Secret".
The classification "is significant because it means that the unauthorised disclosure of those emails could result in exceptionally grave damage to national security," the agent wrote.
In the Weiner probe, the FBI discovered header information on emails pointing to correspondence to and from Mrs Clinton, the papers say.
Given the findings of the earlier probe, "there is also probable cause to believe that the correspondence ... contains classified information which was produced by and is owned by the US government," they say.
US District Judge P Kevin Castel ruled yesterday that the public had a right to see the search warrant application and supporting court paperwork, but he said portions would be blacked out to protect the identity of the investigating agents.
The judge also ordered the redaction of sections of the paperwork related to the ongoing investigation into Mr Weiner's online correspondence.
The discovery of the emails on the disgraced former New York congressman's laptop prompted FBI Director James Comey to briefly reopen an investigation that he had closed over the summer into Mrs Clinton's use of the private server to handle emails she sent and received as secretary of state.
After getting court consent to delve into the newly discovered emails on October 30, agents spent several days analysing them before Mr Comey announced that they contained no new evidence of any wrongdoing by Mrs Clinton.
E Randol Schoenberg, a Los Angeles lawyer who had sued to obtain the court papers, said he saw "nothing at all in the search warrant application that would give rise to probable cause, nothing that would make anyone suspect that there was anything on the laptop beyond what the FBI had already searched and determined not to be evidence of a crime, nothing to suggest that there would be anything other than routine correspondence between Secretary Clinton and her long-time aide Huma Abedin."