One of the businessmen accused of conspiring with associates of Rudy Giuliani to make illegal campaign contributions argued Thursday in federal court that the government is trying to cover up the "warrantless and unlawful electronic surveillance" it conducted in its investigation, including possible "stingray" technology to track cellphones.

Saying it now "appears to be the case" that the government obtained evidence through the use of the Foreign Intelligence Surveillance Act (FISA) and that it was "inconceivable" that no communications were intercepted, lawyers for Andrey Kukushkin sought an explanation as to the "genesis of such evidence and the extent to which it was used."

The bombshell filing comes as the FISA court has sternly rebuked the FBI for submitting false and incomplete materials as part of its investigation into other members of the Trump team. However, the FISA court has not sought a comprehensive review of past FBI filings, and at least one high-level FBI agent apparently involved in FISA misconduct remains at the bureau.

"The government's insistence on complete secrecy is incompatible with the rights of the defendants, particularly in the face of rapidly advancing technology," Kukushkin's attorneys wrote Thursday. "Initially, it was the government's repeated insistence that 'no Title III [wiretap] warrants were used in this investigation,' even when asked whether non-Title III surveillance occurred. This was coupled by the government's refusal to answer direct and repeated questions as to its use of stingray and other electronic surveillance."

The attorneys continued: "Indeed, other than Title III warrants, there is not a single form of surveillance that the government has denied using in the course of this investigation, which makes sense and proves the defense's point."

The government, they said, should be ordered to undertake a comprehensive "agency search" and "affirm or deny" the existence of such evidence.

READ KUKUSHKIN'S LATEST FILING ; READ INITIAL DECEMBER FILING SEEKING INFO

Kukushkin's legal team referenced the FBI's well-known FISA problems, saying in their filing with the Southern District of New York that recent events have proven that the government "routinely" surveils U.S. citizens without judicial review. In this case, they said, it's legally insufficient for the government to declare only that it did not actually use FISA evidence in its criminal probe into Kukushkin, and then attempt to shut down further inquiries.

The government's lack of denials came after a December filing from Kukushkin's attorneys seeking information.

It remained possible, the attorneys argued, that the government was engaging in a prohibited tactic known as "parallel construction" -- meaning that the FBI may have illegally used FISA surveillance, then sought to reobtain that evidence through permissible means. (The government is ordinarily only allowed to use illegally obtained evidence if it can show that it would have found the evidence anyway, without any need for the initial, illegally obtained materials.)

Additionally, it could be that the FISA materials included exculpatory evidence that the government would be obligated to turn over, the attorneys said.

"The government does not deny a single factual contention made by the defense," the attorneys said. "Nor do these facts exist in a vacuum. We submit they must be considered in the context of the extensive surveillance of foreign and United States persons that is undertaken daily by the government, in particular, law enforcement queries of warrantless interceptions as a matter of course when criminal investigations are initiated."

WHO IS AGENT PIENTKA, AND WHY IS THE FISA COURT NOT INTERESTED IN HIS APPARENT MISCONDUCT?

Kukushkin and David Correia pleaded not guilty in October to federal charges that they conspired with two associates of Giuliani to make illegal campaign contributions to prominent Republican politicians they thought could boost their political and business interests. Each man faces up to five years in prison if convicted of the charge against him.

Prosecutors allege that Correia and Kukushkin worked to conceal political donations from an unnamed foreign national who was funneling the money through a shell company, Global Energy Producers (GEP). The company was created by Giuliani's associates, Lev Parnas and Igor Fruman, to support a recreational marijuana business venture, according to the 21-page indictment filed in the Southern District of New York.

Parnas and Furman were hired by Giuliani to try and persuade Ukrainian officials to investigate former vice president and 2020 Democratic primary candidate Joe Biden and his son Hunter Biden for possible corruption. That effort led to Trump's impeachment last year.

The foreign national referred to in the indictment also allegedly wired $500,000 to Parnas and Fruman, both U.S. citizens born in the former Soviet Union, from a foreign bank account to be used to support candidates in state and national elections in Nevada, New York and elsewhere.

Federal prosecutors say that Parnas donated $20,000 to an unidentified congressman in exchange for the politician's support for ousting then-U.S. Ambassador to Ukraine Marie Yovanovitch at the behest of Ukrainian government officials.

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The indictment against the four men does not mention the politician by name, simply referring to the person as "Congressman 1," but donations made by Parnas and Fruman match campaign finance reports for former Rep. Pete Sessions, R-Texas, who has since contributed his donations to local charities.

Fox News' Marta Dhanis and Vandana Rambaran contributed to this report.