The US government charged Friday that the Russian government, possibly at the highest level, had directed recentcompromises of email systems used by US political organizations.
The intrusions include a well-publicized hack of Democratic Party emails in July exposed favouritism within the top echelons of the party for Hillary Clinton over rival Bernie Sanders.
The hack was exposed by several websites, including WikiLeaks, on the eve of the Democratic National Convention in July.
It backed complaints by Sanders, runner-up for the left-leaning party's presidential nomination, that the system was "rigged" against him. The scandal also led to the resignation of party chairwoman Debbie Wasserman Schultz.
The Department of Homeland Security (DHS) and director of national intelligence issued a joint statement Friday saying the US Intelligence Community "is confident" that the Russian government directed the recent attacks, including the one against US political organizations.
"These thefts and disclosures are intended to interfere with the US election process," the statement said.
It said US intelligence and security officials believed "based on the scope and sensitivity of these efforts" that only Russia's senior-most officials could have authorized the cyber attacks.
Within weeks of the attack on the Democratic National Committtee, the Democratic Congressional Campaign Committee (DCCC), which aids lawmakers in their election campaigns, reported that it also fell victim to an attack.
Russia was suspected from the beginning; the Kremlin denied the allegation.
Homeland Security previously said the disclosures through websites such as WikiLeaks were "consistent with the methods and motivations of Russian-directed efforts," and similar to Moscow's interference in politics elsewhere.
Friday's statement said the Russians had used similar tactics and techniques across Europe and Eurasia to influence public opinion there.
The statement also said some US states had recently seen "scanning and probing" of their election-related systems, which in most cases originated from servers operated by a Russian company.
But the federal authorities "are not in a position" to attribute the activity to the Russian government, the statement said.
The statement said it would be extremely difficult for someone, including a "nation-state actor," to alter actual ballot counts or election results by launching cyber attacks or intrusions.
US elections are based on a decentralized format, and states ensure that voting machines are not connected to the internet, the agencies said. Additionally, there are numerous checks and balances built in to the process.
Late Friday, Wikileaks issued another 2,200 emails connected to the Clinton campaign. Media reports said some of the emails suggest political links between the Clinton campaign, her family's Clinton Foundation charity, and the State Department's 2010 approval of the takeover of a US nuclear industry firm by Russian government-controlled interests.