The computer network used by Democratic presidential candidate Hillary Clinton's campaign was hacked as part of a broad cyber attack on Democratic political organisations, people familiar with the matter have told Reuters.
The latest attack, which was disclosed to Reuters on Friday (local time), follows reports of two other hacks on the Democratic National Committee and the party's fundraising committee for candidates for the US House of Representatives.
The US Department of Justice national security division is investigating whether cyber hacking attacks on Democratic political organisations threatened US security, sources familiar with the matter said.
The involvement of the Justice Department's national security division is a sign that the Obama administration has concluded that the hacking was state sponsored.
It was not immediately clear what information on the Clinton campaign's computer system hackers would have been able to access.
Hackers, whom US intelligence officials have concluded were Russian, gained access to the entire network of the fundraising Democratic Congressional Campaign Committee, or DCCC, said people familiar with the matter, detailing the extent of the breach to Reuters for the first time.
Access to the full DCCC network would have given the hackers access to everything from emails to strategy memos and opposition research prepared to support Democratic candidates in campaigns for the House.
The hack of the DCCC, which is based in Washington, was reported on Thursday (local time) before Ms Clinton's speech in Philadelphia accepting the Democratic Party's nomination.
Several US officials said the Obama administration has avoided publicly attributing the attacks to Russia as that might undermine Secretary of State John Kerry's effort to win Russian cooperation in the war on Islamic State in Syria.
The officials said the administration fears Russian President Vladimir Putin might respond to a public move by escalating cyber attacks on US targets, increasing military harassment of US and allied aircraft and warships in the Baltic and Black Seas, and making more aggressive moves in Eastern Europe.
The hacks under scrutiny did not involve the private email system Ms Clinton used while she was secretary of state.
The FBI said on July 5 that no criminal charges should be filed over Ms Clinton's use of private email servers while she was secretary, but it rebuked her for being "extremely careless" in the handling of classified information.
After emails were leaked from the DNC hack, Republican presidential nominee Donald Trump on Wednesday invited Russia to dig up thousands of "missing" emails from Ms Clinton's time at the State Department, prompting Democrats to accuse him of urging foreigners to spy on Americans.
On Thursday (local time), Mr Trump said his remarks were meant as sarcasm.