(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.
The round of subpoenas and interviews suggests prosecutors are digging more deeply into the financial relationships between the two Florida men, who appeared to rise out of nowhere to become fixtures at Trump fundraisers and fixers for Giuliani's efforts in Ukraine.
Ballard did not respond to requests for comment. An executive at the Ballard firm declined to comment.
One of the subpoenas was sent to Paul Okoloko, an executive with a Nigerian fertilizer company who lives in Florida, one source said. Parnas pitched Okoloko to invest in Fraud Guarantee, the same company that ultimately received a $500,000 investment from a plaintiffs lawyer that was paid to Giuliani. A spokesman for Okoloko did not have an immediate comment.
Another subpoena, according to the source, was sent to Meredith O'Rourke, a prominent Florida GOP fundraiser who worked with the Trump Victory Fund. O'Rourke did not respond to a request for comment.
Ballard Partners paid Parnas $45,000 in 2018. A person familiar with the transaction previously told CNN it was because Parnas had made client referrals. The person said no payments have been made in 2019.
Ballard represented the Republic of Turkey in 2017 and 2018, according to foreign lobbying filings made with the US Department of Justice.
Kelly Sadler, a spokeswoman for America First Action, told The New York Times, which first reported the subpoenas, that the group contacted the Southern District of New York last month "and offered to voluntarily cooperate." She added that it had not been subpoenaed.
"America First Action strictly follows the law and, as this is an open matter, we have no further comment," Sadler said.
A spokesman for the US attorney's office in Manhattan declined to comment.