A June 21, 2016 Bloomberg News article claims the warnings came before the hack on the DNC [Democratic National Committee] was made public on June 14, 2016. However, it’s unclear when the warnings happened exactly. This is according to one unnamed “person familiar with the government investigation into the attacks.”
But the Trump campaign won’t respond to questions about the warnings, and Sanders spokesperson Michael Briggs says he isn’t aware of the warnings.
Bloomberg News will comment, “Information about the scope of the attacks and the government warnings raises new questions about how long the campaigns have known about the threats and whether they have done enough to protect their systems.” (Bloomberg New, 6/21/2016)
It has been reported that the Clinton campaign and related organizations have been attacked by hackers, but there have been no confirmed attacks on the Trump or Sanders campaigns. (Bloomberg News, 6/17/2016)