Washington/Philadelphia: The Federal Bureau of Investigation is investigating the nature and scope of a cyber intrusion at the Democratic National Committee, the agency said on Monday, amid concerns hackers working for Russia are attempting to use the breach to influence the US presidential election.
"A compromise of this nature is something we take very seriously, and the FBI will continue to investigate and hold accountable those who pose a threat in cyberspace," the FBI said in a statement.
Emails among DNC employees were released by anti-secrecy group WikiLeaks over the weekend appearing to expose favouritism for presumptive Democratic nominee Hillary Clinton over her chief rival in the primary contest, Senator Bernie Sanders.
The correspondence prompted the resignation of DNC chairwoman Debbie Wasserman Schultz on Sunday.
Meanwhile, chaos broke out ahead of the convention on Monday as protesters jeered the party chairwoman over leaked emails showing Democratic officials worked to undermine Bernie Sanders in his presidential primary battle with Hillary Clinton.
Hours before the start of the four-day gathering to nominate Clinton for the White House, outgoing Democratic National Committee head Debbie Wasserman Schultz struggled to be heard above boos as she spoke to the Democratic delegation from her home state, Florida.
Protesters held up signs that read "Bernie" and "E-MAILS", as she spoke.
Others at the meeting cheered and clapped for Wasserman Schultz, who is stepping down over the email controversy.
She promised to work hard for a Clinton victory over Republican Donald Trump in the November 8 election.
"We know that the voices in this room that are standing up and being disruptive, we know that is not the Florida we know. The Florida we know is united," the congresswoman shouted over the noise of the crowd.
It was an embarrassing prelude to the convention in Philadelphia, which Democratic officials had hoped would convey no-drama competence in contrast to the volatile campaign of Trump.
The New York businessman was formally nominated for president at a chaotic Republican convention in Cleveland last week.
At least one national opinion poll showed Trump benefiting from a convention "bump" and pulling just ahead of Clinton, having lagged her for months.
While Sanders has endorsed Clinton, she faces the task of winning over his backers as she battles Trump.
His supporters were already dismayed last week when Clinton passed over liberal favourites like US Senator Elizabeth Warren of Massachusetts to select the more moderate U.S. Senator Tim Kaine of Virginia as her vice presidential running mate.
"They throw 'party unity' around as if we're supposed to jump for joy when they mention her name," said Manuel Zapata, a Sanders delegate from California, referring to Clinton."What we've been saying for months is obviously true: they had the finger on the scale of the campaign," he said.
Trump gloated at the Democrats' opening day disorder.
"Wow, the Republican Convention went so smoothly compared to the Dems total mess," he wrote on Twitter.
The Republican gathering was overshadowed by accusations of plagiarism in a convention speech by Trump's wife, Melania, and by former presidential candidate Senator Ted Cruz's angry refusal to endorse Trump.
A CNN/ORC opinion poll on Monday gave Trump a three-point lead over former secretary of state Clinton, 48 per cent to her 45 per cent in a two-way presidential matchup.
The survey was conducted July 22-24 and had a margin of error of 3.5 percentage points.
Clinton, 68, a former first lady and US senator, will be the first woman nominated for president by a major US political party.
She waged a months-long battle to defeat the unexpectedly tough challenge from Sanders, 74.
Norman Solomon, an influential Sanders delegate, said there was deep unhappiness among supporters of the Vermont senator in Philadelphia with parts of the Democratic platform and with Clinton's vice presidential pick.
"There's very strong support for a challenge for a Tim Kaine nomination," he told a news conference.
Solomon said some of the 1,900 Sanders delegates in Philadelphia were discussing staging a protest on the convention floor during Clinton's nomination acceptance speech on Thursday, although it was unclear what the protest might be.
Sanders was among those due to speak on the first evening of the convention.
Others included President Barack Obama's wife, Michelle Obama.