Federal investigators probing D.C. Mayor Vincent Gray's campaign are reportedly exploring whether campaign staff members promised prospective donors that their contributions would go unreported.

In a story published Wednesday morning, the Washington Post said campaign aides allegedly offered guarantees of anonymity to city contractors and employees who wanted to support Gray's campaign but did not want to expose themselves to retaliation from incumbent Mayor Adrian Fenty.

The Post's story was based on interviews with unnamed sources, though an attorney for the president of the D.C. Chamber of Commerce acknowledged that agents had interviewed Barbara Lang about her contacts with Gray campaign officials.

Gray told the newspaper that he was "incredulous" about the alleged promises, as well as the possibility that dozens of contributions to his campaign were left off disclosure reports.

Authorities have been investigating the Gray campaign since March 2011 and opened their probe after a minor mayoral candidate accused the campaign of paying him to remain in the race to pester Fenty during events.

That probe has netted guilty pleas from three people this year. In May, two campaign officials acknowledged their roles in the payments to Sulaimon Brown, while a third person pleaded guilty in July to helping organize an illicit "shadow campaign" to benefit Gray.

The mayor has repeatedly denied wrongdoing and has vowed to finish his term, which ends in January 2015.