Hillary Clinton's presidential campaign has been hacked as part of a larger cyber attack against U.S. Democratic Party institutions, U.S. media reported on July 29.
The latest hack follows two data breaches involving the Democratic National Committee and the Democratic Congressional Campaign Committee.
U.S. intelligence officials and private experts believe that the cyber attacks were committed by agents working for the Russian government. The attackers left traces that identify them as Russian and have used tools only available to governments.
Some fear that Russia may be trying to influence the presidential election -- a possibility that U.S. President Barack Obama raised this week.
The Russian government has strongly denied the charges, describing them as "poisonous" and a symptom of "anti-Russian paranoia" that often engulfs U.S. presidential campaigns.
The Clinton campaign said an analytics data program had been accessed by hackers, but there was no evidence that its internal systems, including e-mail, were compromised.
The FBI said it was investigating the allegations and the extent of any hacking.
Hacked e-mails from the Democratic National Committee were leaked last week in an embarassment for the party right before its widely watched presidential nominating convention.