WASHINGTON (Reuters) - Former White House national security adviser Michael Flynn declined on Monday to comply with a subpoena from the Senate Intelligence Committee as it investigates possible Russian interference in the 2016 U.S. election.

Flynn invoked his Fifth Amendment constitutional protection against self-incrimination, according to a letter to the Senate committee from his attorney, which was seen by Reuters.

Flynn’s attorneys did not respond to multiple requests for comment.

The retired lieutenant general is a key witness in the Russia probe, which has roiled the first months of President Donald Trump’s presidency with a spate of negative news reports.

Senators Richard Burr and Mark Warner, the top Republican and Democrat on the intelligence panel, said in a statement they were disappointed by Flynn’s decision, but would “vigorously pursue” his testimony and documents related to the investigation.

The committee is conducting one of the main congressional probes into U.S. intelligence agency reports of Russian meddling in the election and whether there was collusion between Trump’s campaign and Russia. Moscow has repeatedly denied the allegations and Trump denies any collusion.

Separately on Monday, the Washington Post reported that Trump asked two of the country’s top intelligence officials, Director of National Intelligence Dan Coats and National Security Agency Director Admiral Michael Rogers, to help him deny any collusion between his team and Russia during his presidential campaign.

Coats and Rogers declined the request, the Post reported.

PENTAGON MISLED?

Flynn apparently misled Pentagon investigators about his foreign connections when he sought to renew his security clearance in early 2016, according to a document obtained by congressional Democrats and released in part on Monday.

Interviewed as part of the clearance renewal process, Flynn said all of his foreign trips as a private citizen “were funded by U.S. companies,” according to excerpts of a March 14, 2016, report compiled by security clearance investigators.

FILE PHOTO - National security adviser General Michael Flynn arrives to deliver a statement during the daily briefing at the White House in Washington U.S., February 1, 2017. REUTERS/Carlos Barria/File Photo

In fact, a trip Flynn made to Moscow in December 2015, where he attended a gala dinner and sat next to Russian President Vladimir Putin, was paid for by Russia Today, which U.S. officials consider a state-run propaganda arm, according to documents previously released by the House Oversight Committee.

The document is the latest to shed light on how Flynn received a clearance and was hired as Trump’s national security advisor. He was forced to resign in February after less than a month for failing to disclose the content of his talks with Sergei Kislyak, Russia’s ambassador to the United States, and misleading Vice President Mike Pence about the conversations.

Excerpts were released by Representative Elijah Cummings, ranking Democrat on the House committee.

Flynn’s decision to decline to comply with the Senate Intelligence Committee subpoena was first reported by the Associated Press.

His attorney wrote to the committee that “the context in which the Committee has called for General Flynn’s testimonial production of documents makes clear that he has more than a reasonable apprehension that any testimony he provides could be used against him.”

Flynn’s legal team said he was rejecting the subpoena because the committee spurned his offer, made by the retired Army general in a May 8 letter, “to give a full account of the facts and to answer the committee’s questions, should the circumstances permit, including assurances against unfair prosecution. We stated that, absent such assurances, General Flynn would respectfully decline your request for an interview and for production of documents.”

NEXT STEPS UNCLEAR

It was not clear what the committee would do if Flynn decided not to comply.

Burr and Warren did not elaborate. Several panel members said they were waiting to learn more before coming to any conclusion about what might come next.

“We’ll see what the future brings,” Democratic Senator Dianne Feinstein told reporters, adding: ” This may portend that (Flynn) faces something serious.”

Senator James Lankford, a Republican panel member, said Flynn was within his rights to invoke the Fifth Amendment of the U.S. Constitution.

“We will get to the truth one way or another,” Lankford said on Twitter. “We need facts, not speculation & anonymous sources.”

A Congressional Research Service report outlined three main options if Congress decides to enforce the subpoena: seeking criminal prosecution through the executive branch, asking the courts for a civil judgment and using a dormant power of “inherent contempt” to detain and imprison an individual.

The latter option has not been used in 75 years. Congress has more often relied on the criminal contempt statute recently.

Reuters reported on Thursday that Flynn and other advisers to Trump’s campaign were in contact with Russian officials and others with Kremlin ties in at least 18 calls and emails during the last seven months of the U.S. presidential race.

Two other former Trump associates - one-time campaign chairman Paul Manafort and Republican operative Roger Stone - have turned over documents the Senate panel had requested, while a third - campaign adviser Carter Page - had not yet complied, NBC News reported, citing a congressional source.

Flynn has acknowledged being a paid consultant to the Turkish government during the campaign.