The FBI said it is investigating how thousands of Democratic National Committee emails were hacked, a breach that Hillary Clinton's campaign maintains was committed by Russia to benefit Donald Trump.
A statement from the FBI confirmed that it is investigating a cyber intrusion involving the DNC, adding that "a compromise of this nature is something we take very seriously."
Wikileaks posted emails on Friday that suggested the DNC was favouring Mrs Clinton over her rival Senator Bernie Sanders during the primary season.
This prompted her campaign to point to a massive hacking of DNC computers in June that cybersecurity firms linked to the Russian government.
Mrs Clinton's campaign chairman John Podesta added fuel to the debate, saying there was "a kind of bromance going on" between Russian president Vladimir Putin and Mr Trump.
The Clinton campaign said Russia favours Mr Trump's views, especially on Nato.
Donald Trump dismissed as a joke claims by Hillary Clinton's campaign that Russia is trying to help him by leaking thousands of emails from the Democratic National Committee.
"The new joke in town is that Russia leaked the disastrous DNC e-mails, which should have never been written (stupid), because Putin likes me," Mr Trump wrote as part of a series of tweets.
"Hillary was involved in the e-mail scandal because she is the only one with judgement (sic) so bad that such a thing could have happened."
The hacking enraged die-hard Sanders supporters who have long claimed that the DNC had its finger on the scale throughout the primaries.
The disclosures prompted the resignation of DNC chairwoman Debbie Wasserman Schultz on the eve of the party's convention in Philadelphia, where Mrs Clinton is expected to officially accept the nomination for president.
It was not immediately clear how WikiLeaks received copies of the internal Democratic emails.
Democratic Party officials learned in late April that their systems had been attacked after they discovered malicious software on their computers.
A cybersecurity firm they employed found traces of at least two sophisticated hacking groups on the Democrats' network - both of which have ties to the Russian government.
Those hackers took at least one year's worth of detailed chats, emails and research on Donald Trump, according to a person knowledgeable of the breach.