NEW YORK — Bernard Madoff’s longtime auditor pleaded guilty to securities fraud charges today, saying he failed to do his job to verify the disgraced money manager’s financial records but did not know Madoff was running history’s biggest Ponzi scheme.

David Friehling, 49, entered the plea in U.S. District Court in Manhattan, apologizing to the thousands of victims who lost billions of dollars while he audited Madoff’s financial records between 1991 and 2008. The plea was part of a cooperation deal with prosecutors.