The FBI warned the Clinton campaign that it was a target of a cyberattack last March, just weeks before the Democratic National Committee discovered it had been penetrated by hackers it now believes were working for Russian intelligence, two sources who have been briefed on the matter told Yahoo News.

In a meeting with senior officials at the campaign’s Brooklyn headquarters, FBI agents laid out concerns that cyberhackers had used so-called spear-phishing emails as part of an attempt to penetrate the campaign’s computers, the sources said. One of the sources said agents conducting a national security investigation asked the Clinton campaign to turn over internal computer logs as well as the personal email addresses of senior campaign officials. But the campaign, through its lawyers, declined to provide the data, deciding that the FBI’s request for sensitive personal and campaign information data was too broad and intrusive, the source said.