FBI agents informed Hillary Clinton's campaign last spring that it had detected attempts to hack into its networks, but the campaign declined to grant an FBI request to examine its internal data, a U.S. official and another source with direct knowledge of the matter tells NBC News.
The FBI warning was first reported by Yahoo News. The sources said the meeting happened in the spring, and that the Clinton campaign turned down the FBI's request for access to its data as part of the investigation into the hack.
A Clinton aide said the FBI did not tell the campaign that it suspected that Russian intelligence agencies were behind the hack attempts. The aide said the campaign had already been aware of the attempted cyber intrusions, and had implemented security steps to protect its networks.
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The Clinton campaign says that while a computer system it used was hacked, there is no evidence he intruders got into internal campaign networks. The campaign declined to comment publicly on the FBI meeting.
Shortly after the meeting, the Democratic National Committee hired a cyber security firm, Crowdstrike, to investigate a potential cyber breach.
Crowdstrike ultimately determined that two Russian intelligence agencies hacked into the DNC, a conclusion shared by U.S. intelligence agencies, senior officials tell NBC News. Another entity, the Democratic Congressional Campaign Committee, was also hacked, the committee has confirmed.
Two sources who have been briefed on the matter tell NBC News that U.S. intelligence agencies see the hacks as part of a much broader campaign of cyber espionage by Russia, one that hit many other targets known to the government and to private security firms but whose names have not been made public.
The question being debated within the U.S. government is whether this amounts to routine spying, or to an attempt by Russia to interfere with the American presidential campaign.
The FBI has not yet determined, officials say, whether Russian intelligence agencies leaked material hacked from the DNC to Wikileaks, which has published emails in recent days that were deeply embarrassing to Democrats.
Related: Could Russian Hackers Spoil Election Day?
CIA Director John Brennan, speaking at the Aspen Security Forum, declined to say who was responsible. "Interference in the U.S. election process is a very, very serious matter, and I think this government will treat it with great seriousness," he said, in response to a question from NBC News.
A senior law enforcement official said it wouldn't be surprising that the campaign would decline to let the FBI examine its internal data, and instead opt to hire a private firm. The official noted that the FBI warned the campaigns of Barack Obama and John McCain in 2008 about hacking attempts. Especially when the suspects are nation-states, the official said, the intent of the warning is to make sure the target firms up its cyber defenses, since the U.S. government rarely seeks to prosecute foreign intelligence hackers.
Still, the disclosure that the FBI warned the campaign may prompt questions about whether Clinton and other Democrats took the hacking attempts seriously enough.
