Federal prosecutors have subpoenaed a longtime aide to former Rep. Pete Sessions Peter Anderson SessionsThe Hill's Campaign Report: New polls show Biden leading by landslide margins The Hill's Morning Report - Presented by Argentum - In Rose Garden, Trump launches anti-Biden screed Pete Sessions wins GOP runoff in comeback bid MORE (R-Texas) as they seek to examine Rudy Giuliani Rudy GiulianiThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting CIA found Putin 'probably directing' campaign against Biden: report Democrats fear Russia interference could spoil bid to retake Senate MORE’s business dealings with Ukraine, including his involvement in efforts to oust the U.S. ambassador to Ukraine.

Caroline Boothe, who served as the chief of staff to Sessions, the former House Rules Committee chairman, notified House Speaker Nancy Pelosi Nancy PelosiDemocratic senator to party: 'A little message discipline wouldn't kill us' Overnight Health Care: New wave of COVID-19 cases builds in US | Florida to lift all coronavirus restrictions on restaurants, bars | Trump stirs questions with 0 drug coupon plan Overnight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds MORE (D-Calif.) that she intends to comply with the subpoena, according to her lawyer.

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“Ms. Boothe notified the Speaker about the subpoena as she was required to do under House Rules. She fully intends to cooperate with the investigation," Elliot Berke, a lawyer at Berke Farah LLP, said in a statement.

A spokesman for Sessions said his client is cooperating with federal investigators.

“Mr. Sessions is cooperating with the US Attorney from the Southern District of New York and will be providing documents to their office related to this matter over the next couple of weeks as requested,” Sessions spokesman Matt Mackowiak said in a statement to The Hill.

Prosecutors with the Southern District of New York (SDNY) are seeking Boothe's testimony after Sessions was referred to as "Congressman-1" in an indictment that alleged that two Giuliani associates, who were recently charged with campaign finance violations, lobbied the GOP lawmaker to help them oust then-U.S. Ambassador to Ukraine Marie Yovanovitch from her post.

Sessions has not confirmed whether he is Congressman-1, but he says he has met multiple times with the two Giuliani associates, Lev Parnas and Igor Fruman. The two allegedly donated to Sessions amid efforts to remove Yovanovitch, but Sessions has denied that he took any official action as a result of meetings that he had with the two Florida businessmen.

Sessions in a statement this month said it was only after "several congressional colleagues reported to me that the current U.S. ambassador to Ukraine was disparaging President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE to others" that he wrote a letter to Secretary of State Mike Pompeo Michael (Mike) Richard PompeoWatchdog confirms State Dept. canceled award for journalist who criticized Trump Trump's push for win with Sudan amps up pressure on Congress Putin nominated for Nobel Peace Prize MORE raising concerns about the diplomat.

“My entire motivation for sending the letter was that I believe that political appointees should not be disparaging the President, especially while serving overseas," he said at the time.

"I have been friends with Rudy Giuliani for more than 30 years,” he said, adding that he does not "know what his business or legal activities in Ukraine have been.”

A spokesman for SDNY did not return a request for comment.

Sessions is said to also be under subpoena to provide documents and other information to federal investigators, according to reports, as they seek to examine whether there was a scheme to funnel foreign money to U.S. politicians.

The subpoena also reportedly is seeking information about Giuliani and his two business associates, who had been helping Giuliani investigate Democratic presidential candidate and former Vice President Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE before their arrest.

House Democrats are also examining the circumstances surrounding the Trump administration's contacts with Ukraine, particularly whether Trump and Giuliani withheld nearly $400 million in financial aid to Ukraine as leverage to get a commitment from Ukrainian President Volodymyr Zelensky to investigate Biden, one of his top 2020 political rivals, as well as interference in the 2016 election.

Multiple witnesses have testified behind closed doors in recent weeks, voicing disappointment and frustration about Yovanovitch's sudden removal.

Yovanovitch earlier this month also delivered damning testimony in a nearly 10-hour closed-door meeting before House investigators, accusing top Trump officials of staging "a concerted campaign” against her based on "unfounded and false claims by people with clearly questionable motives.”

“I do not know Mr. Giuliani’s motives for attacking me,” she said, according to her prepared remarks. “But individuals who have been named in the press as contacts of Mr. Giuliani may well have believed that their personal financial ambitions were stymied by our anti-corruption policy in Ukraine.”