Washington, D.C. Mayor Vincent C. Gray rejected a proposal by federal prosecutors in September that he plead guilty to a single felony count in connection with their long-running investigation into his 2010 campaign, a person with knowledge of the talks told The Washington Post. Prosecutors have re-interviewed key witnesses in recent weeks, several individuals familiar with the case said—indicating authorities are marching toward an indictment of the outgoing mayor. A deputy to U.S. Attorney Ronald C. Machen Jr. offered the plea deal in a mid-September meeting with Gray’s attorney, Robert S. Bennett. The deal would have required Gray to admit to one felony count. Bennett told the prosecutor that Gray would not accept any deal requiring him to plead guilty to anything. Six people have pleaded guilty to federal crimes related to Gray’s mayoral campaign since 2012—most recently Mark H. Long, the mayor’s campaign driver, who admitted in September to conspiring to violate campaign-finance laws.