As pressure mounts on District of Columbia Mayor Vincent Gray to explain criminal activity that occurred during his 2010 campaign, an Associated Press review has found that the campaign routinely paid day laborers $100 in cash — twice the legal limit — to tout Gray outside polling places.

The payments were later referred to on campaign finance reports as "consulting fees" given to campaign staff members and volunteers, the AP review found.

The FBI and the U.S. Attorney's Office have been investigating Gray's campaign for more than a year. Three campaign aides to the Democratic mayor have pleaded guilty to financial misconduct, including a public-relations executive who admitted this week that she facilitated the use of $650,000 in illicit funds from a district businessman to support the campaign.

Gray has long maintained he had no knowledge of any wrongdoing, and he has not been accused of a crime. Three members of the D.C. Council are calling on Gray to resign, although the mayor says he has no plans to do so.