Washington: The FBI announced Monday it was investigating the embarrassing hack of Democratic National Committee emails — a breach the Clinton campaign blamed on Russia, accusing Moscow of seeking to influence the US presidential election.
The emails leaked by WikiLeaks, which reveal that party leaders sought to undermine the campaign of Hillary Clinton's rival Bernie Sanders, threw the Democratic National Convention into disarray on its first day and prompted the party boss to resign.
While a series of experts pointed the finger at Moscow, others urged caution. Russia denied any involvement.
"The FBI is investigating a cyber intrusion involving the DNC (Democratic National Committee) and are working to determine the nature and scope of the matter," the agency said, making no mention of possible culprits.
"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."
Both the White House and the State Department deferred to the FBI on whether Russia was to blame, but highlighted that cyber security has been an ongoing issue of concern between Washington and Moscow.
"We know that there variety of actors both state and criminal who are looking for vulnerabilities in the cyber security of the United States and that includes Russia," White House spokesman Josh Earnest said.
State Department spokesman John Kirby added: "I think we need to let the FBI do their work before we try to form any conclusions here about what happened and what the motivation was behind it."
Russian Foreign Minister Sergei Lavrov batted away any suggestion that Moscow was behind the hack. He spoke ahead of a meeting Tuesday with US counterpart John Kerry in Laos.
Lavrov, in the Laos capital Vientiane for a regional security forum, shrugged when asked by reporters if Russia was responsible.
"Well I don't want to use four letter words," he said cryptically, before greeting Kerry with a handshake.
Kerry said later, after his talks with his Russian counterpart: "With respect to Foreign Minister Lavrov, I did raise the issue of DNC. As you know the FBI is investigating the incident and it's important for the FBI to do its work before we draw any conclusion."
Was it Russia?
But Clinton's team — looking to tamp down an internal party uproar just days before she becomes the first woman in US history to be formally conferred the presidential nomination by a major party — was quick to point fingers.
"It's troubling that some experts are now telling us that this was done by the Russians for the purpose of helping Donald Trump," Clinton campaign manager Robby Mook told ABC.
Trump has made no secret of his admiration for Russian President Vladimir Putin, leading some to conjecture that Putin was working to put the real estate billionaire in the White House.
The question of Russian involvement has some precedent.
When security firm CrowdStrike revealed that it responded to a suspected DNC breach, uncovered in April, the company identified "two sophisticated adversaries" whom it linked to Russian intelligence.
The Washington Post reported that the hackers stole data including a trove of opposition research on Trump.
Thomas Rid, author of the book "Rise of the Machines" and professor of security studies at King's College, London, believes there is "very strong" evidence linking known Russian operations to the DNC breach.
"The operation, with its manipulative traits, fits well into the wider framework of Russia’s evolving military doctrine, known as New Generation Warfare," he wrote on news website Motherboard.
"American inaction now risks establishing a de facto norm that all election campaigns in the future, everywhere, are fair game for sabotage," he said.
Russian hack, but what about leak?
Trump sought to use the internal Democratic rift to his advantage.
"She worked very, very hard to rig the system," Trump said of Clinton at a campaign stop in Virginia on Monday. "Little did she know that China, Russia, one of our many, many friends came in and hacked the hell out of us."
But even if Russians were involved in the hack, they were not necessarily behind the leak, warned James Lewis, a cybersecurity expert at the Center for Strategic and International Studies think tank.
"It could just as easily be an insider using the Russians as an excuse," he told AFP in an email.
Some experts counselled even greater caution.
"Internet attack attribution is very hard, and in many cases impossible," said Bruce Schneier, cyber-security expert and chief technology officer at Resilient, an IBM company. "We'll probably never know who did this."
WikiLeaks founder Julian Assange told NBC News that there was "no evidence" so far that Russia was behind the leak, but would not elaborate on how the anti-secrecy website obtained the nearly 20,000 DNC emails.