An investigation into the office of New Jersey governor Chris Christie has been broadened to include allegations his deputy held Hurricane Sandy relief funds “hostage”, for approval on a redevelopment plan favoured by the governor.

Christie, who is seen as a potential Republican presidential candidate in 2016, is already subject to an investigation and lawsuits concerning the allegedly politically motivated closure of one of the world’s busiest bridges, the George Washington bridge between New Jersey and New York.

On Saturday, the Hoboken mayor, Dawn Zimmer, added further fuel to the political fire by alleging that in May 2013 the lieutenant governor, Kim Guadagno, threatened to hold back aid related to damage caused by Sandy, unless Zimmer supported a proposed development by the Rockefeller Group.

On Sunday, the New Jersey assemblyman John Wisniewski said his special legislative panel – one of a number of investigations looking into the actions of Christie's administration – would look into Zimmer’s claims as well as those regarding the George Washington bridge. A federal investigation is looking into allegations that Christie misused Sandy relief funds for an advertising campaign that put him in the spotlight in an election year.

"Clearly the allegation that she was asked to support a redevelopment project where there was funding from the Port Authority – which we are investigating – in return for her getting money for her municipality raises serious allegations," Wisniewski told NBC’s Meet the Press. "We don't know where it leads, we don't know if there is more to it but I think it's something the committee has to consider as part of the overall investigation."

Zimmer, a Democrat, has previously been a vocal supporter of Christie. In extracts from her personal journal released to MSNBC, she wrote: “I thought he was honest, I thought he was moral – I thought he was something very different. This week I found out he’s cut from the same corrupt cloth that I have been fighting for the last four years. I am so disappointed it literally brings tears to my eyes.”

Interviewed on CNN on Sunday, Zimmer said her number one priority had been to secure as much funding for Hoboken after Sandy, which hit the eastern US in October 2012. The storm flooded half the city and crippled transportation.

“Back in May when the lieutenant governor came to me and said these two things are connected I didn’t think anyone would believe me,” she said, when asked why she had not spoken out until now. “I really didn’t. Quite frankly if I came forward and no one believed me, I am going to put Hoboken in an even worse position.”

She said Guadagno had made a “direct threat” to her about funding being tied to the Rockefeller project: “When the lieutenant governor comes and pulls you aside in a parking lot and says these two things are connected, [and says] 'I know it shouldn’t be but they are and if you tell anyone about it, I’ll deny it' … She felt almost guilty about saying it. She knows it’s wrong.”

Reached by reporters from New Jersey’s Star Ledger newspaper, Guadagno declined to comment on Zimmer’s accusations.

Colin Reed, a spokesman for Christie, told CNN: “It’s very clear partisan politics are at play here as Democratic mayors with a political axe to grind come out of the woodwork and try to get their faces on television.”

Zimmer answered: “I am offering to testify under oath. What are they doing? They are hiding behind spokespeople. It’s stunning. It’s outrageous. But it’s true.”

Rockefeller officials released a statement on Saturday, denying any knowledge of the arm-twisting alleged by Zimmer.

“We have no knowledge of any information pertaining to this allegation,” the statement said. “If it turns out to be true it would be deplorable.”