Former National Security Adviser Michael Flynn invoked his Fifth Amendment rights on Monday, as his lawyers refused to honor a subpoena for documents from a U.S. Senate committee investigating Russian interference in the 2016 elections, the first time that someone with close ties to President Trump has refused to cooperate in the course of this politically charged investigation.

"In these circumstances, General Flynn is entitled to, and does, invoke his Fifth Amendment privilege against production of documents," wrote Flynn's lawyers in a letter to the Senate Intelligence Committee, which had sought evidence and testimony from Flynn.

"He is the target on nearly a daily basis of outrageous allegations, often attributed to anonymous sources in Congress," the letter stated, decrying an "escalating public frenzy against him."

Flynn's lawyers also cited the appointment last week of a special counsel to the probe into Russian influence in 2016 as reason to withhold testimony at this time.

The decision by Flynn did not surprise committee members; last week, panel chairman Sen. Richard Burr (R-NC) had told reporters that Flynn was not going to honor their subpoena for documents on meetings and communications with "any Russian official."

"While we recognize General Flynn's constitutional right to invoke the Fifth Amendment, we are disappointed that he has chosen to disregard the Committee's subpoena request for documents relevant to our investigation," said Burr, and top committee Democrat Sen. Mark Warner (D-VA).

"We will vigorously pursue General Flynn's testimony," Burr and Warner added in a statement.

It was not immediately apparent what the Intelligence Committee could do to compel Flynn to either testify, or turn over documents.

One option is holding Flynn in contempt of Congress - but that does not guarantee cooperation of a witness, either.

During the 2016 campaign, Flynn himself had made light of people who had taken the Fifth in the investigation of Hillary Clinton's email server, as well as those who were granted immunity.

"When you're given immunity, that means you probably committed a crime," Flynn told NBC's Meet the Press on September 25, 2016.

Democrats not only dug up old quotes of Flynn, but also some from President Trump, where he also raised questions about those same aides with ties to the Clinton email server.

"If you're innocent, why are you taking the Fifth Amendment?" Mr. Trump asked at a rally in Iowa last September.

Flynn has come under scrutiny for several things – his contacts with Russian officials during the Trump transition, not disclosing payments from Russian groups in 2015 as required for former top military officers, and belatedly disclosing that he was working as a paid agent of the Turkish government, even as he was campaigning for Mr. Trump last year.

In Congress, Democrats today pressed Republicans to subpoena White House documents related to Flynn, as Rep. Elijah Cummings (D-MD) said it appeared that Flynn had lied to investigators when his security clearance was renewed in 2016, by saying he had been paid by "U.S. companies" when he went to Moscow in late 2015.

Documents later showed that Flynn was paid over $45,000 by RT, a Kremlin-backed television network.