In fact, the available forensic evidence contradicts the official account that blames the leak of the DNC emails on a Russian internet "intrusion". The existing evidence supports an alternative explanation--the files taken from the DNC between 23 and 25May 2016 and were copied onto a file storage device, such as a thumb drive.
If the Russians actually had conducted an internet based hack of the DNC computer network then the evidence of such an attack would have been collected and stored by the National Security Agency. The technical systems to accomplish this task have been in place since 2002. The NSA had an opportunity to make it clear that there was irrefutable proof of Russian meddling, particularly with regard to the DNC hack, when it signed on to the January 2017 "Intelligence Community Assessment," regarding Russian interference in the 2016 Presidential election:
We also assess Putin and the Russian Government aspired to help President-elect Trump's election chances when possible by discrediting Secretary Clinton and publicly contrasting her unfavorably to him. All three agencies agree with this judgment. CIA and FBI have high confidence in this judgment; NSA has moderate confidence.