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The Kremlin unequivocally denied on Tuesday reports that it was involved in hacking the Democratic National Committee’s emails, even as American intelligence agencies become increasingly convinced that their Russian counterparts did in fact have something to do with it.

“We are again seeing these maniacal attempts to exploit the Russian theme in the U.S. election campaign,” Kremlin spokesman Dmitry Peskov told reporters. “This is not breaking new ground, this is an old trick which is being played again. This is not good for our bilateral relations, but we understand that we simply have to get through this unpleasant period.”

However, investigators believe that the hacker who called himself “Guccifer 2.0,” who leaked a large sampling of the documents to news organizations weeks before the WikiLeaks dump, is in fact an agent of Russia’s military intelligence service, G.R.U. The New York Times reports:

The federal investigation, involving the F.B.I. and the intelligence agencies, has been going on since the Democratic National Committee first called in a private cybersecurity firm, Crowdstrike, in April.

Preliminary conclusions were discussed on Thursday at a weekly cyberintelligence meeting for senior officials. The Crowdstrike report, supported by several other firms that have examined the same bits of code and telltale “metadata” left on documents that were released before WikiLeaks’ publication of the larger trove, concludes that the Federal Security Service, known as the F.S.B., entered the committee’s networks last summer.

The G.R.U., a competing, military intelligence unit, was a later arrival. Investigators believe it is the G.R.U. that has played a bigger role in releasing the emails.

U.S. Secretary of State John Kerry raised the issue with Russian Foreign Minister Sergei Lavrov at a meeting in Laos on Tuesday. Asked by reporters for his response to the allegations, Lavrov said: “I don’t want to use four-letter words.”