Hillary Clinton's presidential campaign has been hacked as part of a larger cyber attack against Democratic Party institutions, US media report.
The latest hack follows two data breaches involving the Democratic National Committee and the Democratic Congressional Campaign Committee.
US officials widely believe that the cyber attacks were committed by agents working for the Russian government.
Some fear that Russia may be trying to influence the presidential election.
The Russian government has denied involvement and denounced the "poisonous anti-Russian" rhetoric coming out of Washington.
The Clinton campaign said on Friday that an analytics data program, which it shared with other entities, had been accessed by hackers.
But, said her press secretary Nick Merrill, there was "no evidence that our internal systems have been compromised".
The FBI said it was investigating the allegations and the extent of any hacking.
Hacked emails from Democratic National Committee were leaked last week on the eve of the party's convention.
The emails showed Democratic Party officials were biased against Bernie Sanders in his primary race against Mrs Clinton.
The hack led to the resignation of the party chairwoman Debbie Wasserman Schultz and sparked protests at the convention in Philadelphia .
In addition to the attack on the Clinton campaign, the Democratic Congressional Campaign Committee (DCCC), was also found to have been compromised.
The DCCC works to raise money for Democratic congressional candidates. They said in a statement early on Friday that they have hired cyber security firm CrowdStrike to help with the investigation.
Hackers may have been able to access the Democrats' political strategy documents as well as opposition research conducted against Republican candidates.
"We have taken and are continuing to take steps to enhance the security of our network," the DCCC said. "We are cooperating with federal law enforcement with respect to their ongoing investigation."