The Democratic National Committee was warned by federal investigators about a potential hack months before the committee sought outside help to look into the problem.
The FBI warned the DNC of the intrusions but the warning was not clear or specific about the extent of the hack, CNN reported .
Donna Brazile, who took over the chairwoman's role after Debbie Wasserman Schultz resigned in the wake of the recent leaked emails scandal, told CNN's Wolf Blitzer that "the general counsel of the DNC today and he assures me that every step along the way when we were notified of these issues that we changed systems, changed procedures."
"But these hackers are so sophisticated that they changed procedures," Brazile said. "So yes, it went on for more than a year, but at no time did we ignore the warning from the FBI or any other federal officials."
Last week, Wikileaks published just over 19,000 emails from the DNC that indicated a clear preference among top officials for Hillary Clinton over her primary rival Sen. Bernie Sanders.
After public criticism and calls from the Sanders campaign, Wasserman Schultz announced her resignation as the head of the DNC on Sunday, and the following day, she decided not to gavel in the opening night of the Democratic National Convention in Philadelphia.