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By Michael Isikoff
Hoboken, N.J., Mayor Dawn Zimmer at a post-Superstorm Sandy news conference in November 2012.Andrew Burton / Getty Images file

FBI agents have begun questioning witnesses in the investigation into whether New Jersey Gov. Chris Christie's aides threatened to cut off Hurricane Sandy relief money to Hoboken unless the city's mayor backed a billion-dollar development project, three sources with direct knowledge of the probe told NBC News on Wednesday.

Federal prosecutors and agents have also instructed key witnesses to preserve all documents and emails relating to the allegations by Hoboken Mayor Dawn Zimmer, these sources said.

After Zimmer gave her account of the alleged threat to federal prosecutors on Sunday, federal agents questioned Dan Bryan, Zimmer's chief of staff, and Juan Melli, her communications director, the sources said.

The two Zimmer aides are among at least five witnesses who Zimmer told the FBI could confirm that she had previously told them about the conversation she says she had with Lt. Gov. Kim Guadagno last May. Zimmer has alleged that during that conversation in a parking lot, the lieutenant governor linked Sandy funding to backing of the development project.

David Mello, a member of the Hoboken City Council, told NBC News that Zimmer told him about the Guadagno conversation last summer as they were walking home after an event. "I distinctly remember [Zimmer] saying that the lieutenant governor said, 'If this came out, she would deny it,'" he said.

"I thought it was absurd and outrageous," said Mello, adding that he pressed Zimmer whether she would speak out publicly. "She told me, 'It was a done deal that [Christie] was going to be re-elected" and she did not want to jeopardize further funding for the city, he said.

Mello, a Zimmer ally who said he strongly opposed Christie's re-election, said he is arranging through the Hoboken city counsel to provide his account to the U.S. attorney's office.

Bryan and Melli declined further comment, citing a request from the office of U.S. Attorney Paul Fishman that city officials not discuss the probe publicly.

Guadagno in a statement to reporters Tuesday adamantly denied Zimmer's account of the May conversation, calling it "false" and "offensive." A Christie press spokesman has also strongly denied Zimmer's account.