The General Intelligence and Security Service of the Netherlands (AIVD) accessed the Russian hacking group known as Cozy Bear as early as the summer of 2014, according to Dutch newspaper de Volkskrant.
According to the report, AIVD no longer has access to Cozy Bear hacker's network, although it is still unclear as to what caused the cyberespionage campaign to come to an end.
Volkskrant and Nieuwsuur said that the Dutch spies used their access to help oust Cozy Bear from U.S. State Department computers in late 2014. "The ally had managed to hack not only the Russians' computers, but also the surveillance cameras inside their workspace, according to the former officials", the Post noted.
Cozy Bear, along with yet another Fancy Bear (APT28) hacking group, are widely considered to be responsible for the cyberattacks targeting the US Democratic Party during the 2016 presidential election.
"Yet, it will be months before the United States realise what this warning means: that with these hacks the Russians have interfered with the American elections".
USA investigators have reportedly been looking into the hacks and the alleged meddling effort at least since they learned Trump adivser George Papadopoulos, who later pled guilty to lying to the Federal Bureau of Investigation, told an Australian diplomat in May 2016 about dirt Russians had on Clinton.
Former FBI Director and current special counsel Robert Mueller just this week indicated he would like to interview Trump as part of the Russian Federation investigation, CNN reported.
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In the past eight months, members of Trump's campaign, family and White House staff have all been interviewed, and four indictments have been issued, with two guilty pleas as the investigation has spiralled closer to Trump himself.
A spokeswoman for the AIVD refused to confirm the Dutch media reports, telling AFP "we never comment on operations".
"If Dutch newspapers want to add fuel to the fire of anti-Russian hysteria that exists in America, this is not the most noble occupation", the Kremlin spokesman pointed out.
Even though the assault was thwarted after a 24-hour cyber battle, one Trojan email did get through, enabling the Russian group to access the White House.
Unmasking the Cozy Bear hackers would provide key evidence for investigators trying to unravel the DNC breach, but it may not dispel the mystery surrounding the leaks that followed.
The Dutch infiltration proved to be "a goldmine", Volkskrant said.