An FBI investigation of last year’s mayoral race includes an examination of Bill de Blasio’s campaign pledge to ban carriage horses, the Daily News has learned.

FBI agents have been questioning people about the pledge, which was made in March 2013, and the ad blitz launched the next month by animal rights advocates attacking de Blasio’s chief rival, Christine Quinn, two sources familiar with the investigation said Friday.

FBI agents also appear highly interested in a $175,000 contribution by a union tied to de Blasio’s cousin, labor leader John Wilhelm, to the animal rights group NYCLASS, the sources Friday.

NYCLASS helped to bankroll the “Anybody But Quinn” campaign attacking Quinn’s mayoral candidacy.

At least five people have been questioned by the FBI in the past month as part of the investigation, those familiar with the inquiry told The News.

The News first disclosed the FBI investigation on Friday, reporting that the FBI was looking at a threat by NYCLASS’ political consultant Scott Levenson early last year to undermine Quinn's campaign if she didn’t back the carriage horse ban.

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