WASHINGTON — Embattled former national security adviser Michael Flynn is asserting his right against self-incrimination in declining to provide a Senate committee documents related to its investigation into possible collusion between associates of President Trump's campaign and the Russian government.

Flynn relayed his decision to invoke the Fifth Amendment to the Senate Intelligence Committee on Monday.  Earlier this month, the panel had issued the formal request for data and communications involving the former Army lieutenant general and Russian officials as part of the committee's ongoing investigation. Senate officials asked Flynn in April turn over the materials voluntarily before issuing the subpoena two weeks ago.

In a letter to the committee, Flynn attorney Robert Kelner wrote that the panel's document request "makes clear that he has more than a reasonable apprehension that any testimony he provides could be used against him."

"Multiple members of Congress have demanded that he be investigated and even prosecuted," Kelner wrote. "He is the target on a nearly daily basis of outrageous allegations, often attributed to anonymous sources in Congress or elsewhere in the United States government, which, however fanciful on their face and unsubstantiated by evidence, feed the escalating public frenzy against him." The contents of the letter, first obtained by the Associated Press, were confirmed by USA TODAY.

Senate Intelligence Committee Chairman Richard Burr, R-N.C., and Sen. Mark Warner of Virginia, the panel's senior Democrat, said they were disappointed by Flynn's decision to refuse the subpoena.

"While we recognize General Flynn's constitutional right to invoke the Fifth Amendment, we are disappointed he has chosen to disregard the committee's subpoena request for documents relevant and necessary to our investigation," the two senators said in a joint statement. "We will vigorously pursue General Flynn's testimony and his production of any and all pertinent materials pursuant to the committee's authorities."

Sen. Dianne Feinstein, D-California, who serves on both the Intelligence and Judiciary committees, called Flynn's decision "unfortunate but not unexpected."

"I believe both the Intelligence and Judiciary committees should continue to seek other ways to gain access to this information," she said, adding that separate requests for data have been made to the White House, FBI and Defense Department. "The investigation will go on."

Sen. James Lankford, R-Okla., a member of the Intelligence committee, said it is "Mike Flynn's right to plead the 5th." The senator tweeted: "We will get to the truth one way or another. We need facts, not speculation and anonymous sources."