Former Trump political adviser Roger Stone has invoked the Fifth Amendment and refused to turn over documents requested by House Judiciary Committee Chairman Jerry Nadler, saying he will not appear on Capitol Hill for an interview as part of the committee’s expansive investigation into President Trump.

Stone delivered his response to Nadler this week, after the chairman sent out a slew of document requests to 81 individuals and entities as part of a sweeping probe into “alleged obstruction of justice, public corruption, and other abuses of power by President Trump.”

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“Mr. Stone respectfully declines to produce any documents, and declines any potential invitation for an interview which may follow,” Stone attorney Grant Smith wrote in a letter to Nadler, obtained by Fox News. “The production of documents that may be responsive to this fishing expedition request would unquestionably be a testimonial act protected by the U.S. Constitution.”

He added that Stone’s “Fifth Amendment rights must be understood by all to be the assertion of a Constitutional right by an innocent citizen, who is currently defending his innocence, and one who denounces secrecy for the purposes of advancing innuendo.”

Stone is facing legal problems on another front. In January, he pleaded not guilty to obstruction of justice, witness tampering, and making false statements to Congress as part of Special Counsel Robert Mueller’s investigation into Russian meddling and potential collusion with Trump campaign associates as part of the 2016 presidential election.

However, he is far from the only individual declining to cooperate with Nadler's probe. The majority of individuals and groups targeted as part of Nadler’s investigation have missed the Monday deadline to respond to document requests, though talks are apparently ongoing. Nadler has said other individuals have agreed to send documents.

But several targets have been vocal about their concerns with the investigation.

WikiLeaks founder Julian Assange, too, has signaled his likely resistance to the requests. Assange’s attorney, Barry Pollack, told Fox News in a statement on Thursday that Assange had “not yet responded to the Committee’s request.”

“The First Amendment dictates that any inquiry by Congress should not begin by issuing requests to journalists for documents pertaining to their news gathering,” he said.

Nadler sent the request to Assange due to WikiLeaks’ move to publish emails stolen from Democrats during the 2016 election.

Former Trump campaign aide Michael Caputo told Fox News that while he was the first of 81 targets to respond, he told the committee that he did not have any of the documents they wanted. He does not plan to cooperate further.

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In addition to Caputo, Nadler sent document requests to Trump family members, like Donald Trump Jr., Eric Trump and Jared Kushner; former administration figures like former chief of staff Reince Priebus, former national security adviser Michael Flynn, former Attorney General Jeff Sessions, and former spokeswoman Hope Hicks; along with Trump campaign figures like Brad Parscale and Corey Lewandowski.

The probe itself cast a wide net, drilling deep into virtually every aspect of Trump's administration and business ventures -- as well as his connections to organizations ranging from the National Rifle Association (NRA) to WikiLeaks.

Nadler's letters to Trump's oldest sons asked questions about events that happened in the White House after their father was elected, including the firing of FBI Director James Comey and discussions surrounding the appointment of Special Counsel Robert Mueller.

Several other people related to the Trump Organization were sent letters, including the Trump Organization itself, Allen Weisselberg, the company's chief financial officer and Rhona Graff, Trump's longtime assistant. Among other matters, the company officials were asked to provide documents regarding "any loan, financing transaction, or capital investment by the Russian Federation, any Russian national, any Russian business, or any other Russian entity to the Trump Organization."

Those who have responded include: former Trump campaign adviser George Papadopoulos; former Trump national security adviser J.D. Gordon; the NRA; Russian lobbyist Rinat Akhmetshin, who attended the infamous 2016 Trump Tower meeting; former Trump political adviser Sam Nunberg; former White House chief strategist Steve Bannon; Trump inaugural committee chair Tom Barrack; and the 58th Presidential Inaugural Committee.

Former White House Communications Director Hope Hicks reportedly will cooperate with the panel’s investigation.