Last month a computer system belonging to the US' Democratic National Committee (DNC), the party's governing body, was hacked, with thousands of emails then released by WikiLeaks, showing how the DNC had sabotaged Bernie Sanders' campaign.
It has now been found that two more computer systems have been hacked, including those used by Hillary Clinton's team.
James Rubin, once a member of Bill Clinton's government and now an informal advisor to Ms Clinton's campaign, says it's most likely the Russians are in the frame.
"Talking to officials in the US government, they have high confidence that this is all part of the same effort to get intelligence information from the Democratic National Committee," he said.
"They have high confidence the Russians are behind this, a similar entity from the same cyber snoops."
He says the hacks have possibly come about as a result of Donald Trump's positions, which he says will "warm the heart "of Russian president Vladimir Putin.
"His suggestion that NATO may not be long in existence, his suggestion the US may not come to the defence of Baltic states for example, and his personal statements about how wonderful he thinks Vladimir Putin is, and his desire to get along well with him," Mr Rubin said.
The mood music might be positive, but Mr Trump says suggestions he and Mr Putin are in cahoots are farfetched. But he makes no apologies for calling on Russia to find and release emails that Ms Clinton deleted from her private account.
"I don't have to be so nice anymore; I'm taking the gloves off, right?" Mr Trump told his supporters days ago.
"Tell Hillary I'm not gonna be nice any more - I've been very nice to her."
US intelligence services are yet to officially confirm Russian involvement in the hacking, but there's no doubt they are taking it extremely seriously.
"Any time there is hacking like that, and the release of proprietary information, it is a crime," CIA director John Brennan said.
"Interference in the US election process is a very, very serious matter, and I think this government will treat it with great seriousness."
Mr Putin has denied any involvement in hacking US computer systems, but his relationship with Ms Clinton has been tense at times in the past.
She has compared Mr Putin to Nazi leader Adolf Hitler, while he said Ms Clinton lacked grace but it was better not to argue with a woman.