Her comments come one day after the FBI arrested a 25-year-old government contractor, Reality Winner of Augusta, Georgia, for allegedly leaking the document to reporters at the online publication The Intercept.
The document, posted online just hours before the announcement of Winner's arrest, laid out in stark detail how Russian hackers allegedly "executed cyber espionage operations" against outside vendors dealing with voter-related information.
It's unclear exactly why Winner allegedly searched for secret documents related to the election, printed out a highly-classified NSA document and then mailed it to a media outlet. But court documents may offer a glimpse.
In late March, Winner allegedly used a Gmail account to contact The Intercept, and she "appeared to request transcripts of a podcast," according to court documents.
Host Jeremy Scahill said "there is a tremendous amount of hysterics" and "a lot of premature conclusions being drawn around all of this Russia stuff," but "there's not a lot of hard evidence to back it up."
At the Senate hearing today, McCaskill said the NSA document allegedly leaked by Winner now offers such evidence, and she pressed the hearing's witness, Homeland Security Secretary John Kelly, to make sure his department takes appropriate steps to protect voting-related systems in the future.
Kelly said he couldn't confirm or deny any specific information "about what actually took place" last year, particularly because the intelligence behind it is so highly classified.
In January, the U.S. intelligence community issued a report calling Russia's alleged meddling in last year's presidential campaign "a significant escalation" of efforts "to undermine the U.S.-led liberal democratic order."
Speaking in Washington last month, however, the NSA's recently-departed deputy director wholly rejected such skepticism, insisting there is "no question it was the Russians."
"NSA had a huge role in making that determination," the former deputy director of the agency, Richard Ledgett, said. "There is no question that that’s what it was. I can’t lay out for you all of the reasons for that, because there's a lot of really sensitive sources that led to that, but it was definitely the Russians."
Ledgett said such conclusions are based on "a variety of different good sources of information."
"It's more than just looking at the code. It's more than just looking at the targets. It's more than looking at the tactics and the techniques and procedures," and U.S. agencies "have a really good ability to do attribution" thanks to the intelligence work of both the U.S. government and allies around the world, he said.
Ledgett, who stepped down from the NSA in April, was speaking at Georgetown Law School’s annual Cybersecurity Law Institute.