Longtime Trump confidant Roger Stone asked Wednesday why lawyers representing Paul Manafort, his old lobbying partner, had not pushed a line of inquiry about whether their client had been subjected to unlawful surveillance by the federal government.

"I'm still perplexed as to why he has not raised the question of whether he was under illegal FISA surveillance in this trial and in the upcoming trial," Stone said during an interview with CNN, referring to Manafort's court case in Virginia and his forthcoming proceedings in Washington. Both legal actions stem from special counsel Robert Mueller's federal Russia investigation.

The Justice Department and FBI have faced ongoing criticism for applications they made to the Foreign Intelligence Surveillance Court to obtain warrants to gather information on another former Trump campaign associate, foreign policy adviser Carter Page.

Stone was yet to hear from Mueller's team of federal investigators but would not testify against Trump, he continued. He would, however, "cooperate" with the Russia investigation if he could.

[Roger Stone: I am 'probably' the unnamed person mentioned in Mueller indictments]

Mueller is reportedly interested in Stone after the self-described political trickster foreshadowed the release of Hillary Clinton campaign chairman John Podesta's emails before a WikiLeaks dump during the 2016 presidential campaign.

Stone on Wednesday added that news WikiLeaks founder Julian Assuage had been asked to appear in a closed-door session before the Senate Intelligence Committee was a "positive development."

Manafort's trial in Virginia regarding mostly tax and bank fraud will enter its eighth day on Thursday.