Moreover, while the F.B.I. did not mention it in court filings, computer security experts noted that the printer appeared to leave barely visible microdots on the printout identifying the serial number of the printer and the date and time of the printing: 6:20 a.m. on May 9, 2017.

Once rare, leak cases have become much more common in the 21st century, in part because of such electronic trails. Depending on how they are counted, the Obama administration brought nine or 10 leak-related prosecutions — about twice as many as were brought under all previous presidencies combined.

The Justice Department prosecuted Ms. Winner under the Espionage Act, a World War I-era law that criminalizes the unauthorized disclosure of national-security secrets that could be used to harm the United States or aid a foreign adversary.

Ms. Winner’s prosecution galvanized transparency advocates, who mounted a publicity campaign in her support that even included a billboard in Augusta, the east Georgia city where Ms. Winner lived at the time of her arrest. They were particularly infuriated by a judge’s ruling that she be held until her trial.

“They’re just coming down on her so tough,” Billie Winner-Davis, Ms. Winner’s mother, said in an interview after Tuesday’s plea hearing was scheduled. “I can only think that it’s because she was the very first one: the one they wanted to make an example out of, the one they wanted to nail to the door as a message to others.”

Still, Ms. Winner-Davis said of her daughter’s willingness to plead guilty, “She wouldn’t have made this decision if she wasn’t ready to accept the consequences and to accept responsibility.”

Ms. Winner is the second person known to have reached a plea agreement with the Trump administration to resolve a leak prosecution. A former F.B.I. agent, Terry J. Albury, pleaded guilty in April, but prosecutors in that case have signaled that they will ask that he serve 46 to 57 months in prison.