A report by Reuters suggests that the FBI was aware of a possibly Russian-sponsored intrusion into the network of the Democratic National Committee as early as last fall. But investigators from the FBI only initially told DNC staff that they should be on the lookout for strange activity on their network—and the feds didn't mention a potential state-sponsored attack until they informed the Clinton campaign in March about a phishing campaign.
Unnamed DNC staffers told Reuters' Mark Hosenball and John Walcott that the FBI had been investigating a potential intrusion into the DNC's network since the fall of 2015. After the initial warning to look for anything suspicious, DNC IT staff checked network logs and scanned files, finding nothing suspicious. When asked to provide more information to help identify a problem, the FBI "declined to provide it," according to the Reuters report.
It was not until March that the DNC IT team realized the severity of the intrusion of their systems, though Reuters did not report what triggered their realization. At about the same time, the FBI reportedly warned the Clinton campaign of the attempted attacks, according to a Yahoo News report. Spear-phishing attacks were detected in March and April against the DNC and the presidential campaign organization of Hillary Clinton by the security company SecureWorks, as Ars has previously reported.