(CNN) Federal prosecutors in New York have subpoenaed several individuals active in President Donald Trump's fundraising machinery as part of their investigation into the associates of Rudy Giuliani, the President's personal attorney, according to people familiar with the investigation.

Prosecutors sent subpoenas in recent weeks to Ballard Partners, a lobbying firm run by Brian Ballard, a top Trump fundraiser, and FBI agents have knocked on the doors of others involved with Republican campaigns, the sources said.

One of the subpoenas asked for communications and documents relating to Lev Parnas and Igor Fruman -- the two Giuliani associates arrested last month on campaign finance charges -- along with a fundraiser at America First Action, a super PAC supporting Trump, and Giuliani himself.

Parnas and Fruman have pleaded not guilty. Giuliani has not been accused of any wrongdoing.

FBI agents questioned some individuals about how they knew Parnas and Fruman, if they knew how the men gained a foothold into fundraising dinners, and how they connected with Giuliani, a person familiar with the matter said.

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