The Russian government has hacked the computer network of the Democratic National Committee (DNC), accessing its entire database of opposition research on Donald Trump, the committee and researchers have said.
The Kremlin hackers had access to the DNC network for over a year, compromising its system so thoroughly that they were able to read all email and chat traffic.
“When we discovered the intrusion, we treated this like the serious incident it is,” said Debbie Wasserman Schultz, the DNC chairman.
“Our team moved as quickly as possible to kick out the intruders and secure our network.”
The Russian hackers also targeted the networks of presumptive presidential nominees Hillary Clinton and Mr Trump, and some other Republican political action committees, US officials said, but provided few details.
The hacking attacks against the DNC focused on the information held in the organization’s research files, which included any dirt its investigators had dug up on Mr Trump.
The DNC told the Washington Post, who first broke the story that no financial, donor or personal information was accessed or taken. Once suspicious of the breach, DNC leaders brought in CrowdStrike, a cyber firm to combat the threat. The cyber security group said it found evidence of hacks by two separate groups.
One, codenamed Cozy Bear, broke into the DNC last summer and had been monitoring the committee’s emails and chats. The other, dubbed Fancy Bear, hacked its servers in April to obtain the opposition research files.
The Kremlin is likely to have already compiled substantial information on Mrs Clinton, given her long political career.
But, analysts said, the attack appeared to signal an effort by Moscow to learn more about Mr trump, who as a real estate mogul with no prior political experience may remain a relatively unknown quantity.
“The purpose of such intelligence gathering is to understand the target’s proclivities,” said Robert Deitz, former senior councillor to the CIA director and a former general counsel at the National Security Agency.
“They may provide tips for understanding his style of negotiating. In short, this sort of intelligence could be used by Russia, for example, to indicate where it can get away with foreign adventurism.”
The Russian embassy in Washington did not immediately respond to a request for comment. A spokesman previously said he had no knowledge of such intrusions.
But on Tuesday night, a Kremlin spokesman denied it was involved in the hack.