President Donald Trump's fired national security adviser has refused to hand over files to a Senate panel probing alleged Russian political meddling.
Michael Flynn invoked his legal right against self-incrimination, his lawyers told the committee.
The Senate Intelligence Committee is investigating possible links between the Trump campaign and Kremlin.
Mr Flynn stood down in February after it emerged he lied about his contacts with the Russian ambassador.
Senate Intelligence Committee chairmen Senators Richard Burr and Mark Warner said they were "disappointed".
But they vowed to "vigorously pursue General Flynn's testimony".
Hours after Mr Flynn's refusal, Democrats said he appeared to have lied when he applied for his White House security clearance.
Elijah Cummings, who sits on the House of Representatives Oversight Committee, said Mr Flynn did not fully disclose a payment he received from a Kremlin-friendly broadcaster.
The Senate Intelligence Committee issued a subpoena - a legal summons - two weeks ago to obtain documents related to his contacts with Russians dating back to June 2015.
Two other former top Trump aides - Paul Manafort and Roger Stone - have complied with the committee's request for information, it was reported on Monday.
There are two congressional inquiries and an FBI investigation into claims that Russian hackers tried to help Mr Trump win last November's presidential election, and into whether members of his campaign colluded with the alleged Kremlin conspiracy.
Mr Flynn's name has cropped up repeatedly in the matter, but his letter to the Senate panel emphasises his refusal to tesitfy is not an admission of wrongdoing.
The former Army lieutenant general is invoking the fifth amendment to the US constitution, which protects Americans from being legally compelled to testify against themselves in a criminal case.
The letter said his decision was a response to the current political climate and an "escalating public frenzy against him".
His attorneys argued that "any testimony he provides could be used against him".
If Mr Flynn continues to refuse to comply, it is thought Senate investigators could vote to hold him in contempt of Congress, or even refer his case for possible criminal charges.
His legal representative has previously demanded immunity from "unfair prosecution" before his client testifies.
Last week the committee's chairman, Senator Richard Burr, told reporters that Mr Flynn was "not co-operating" with the investigation.
Shortly after Mr Flynn left the White House, the Department of Defense also launched an inquiry into payments he received for a speech in Moscow to media outlet RT, and for lobbying on Turkey's behalf.
Mr Flynn misled the White House about discussing US sanctions against Russia with Moscow's envoy, Sergei Kislyak, before Mr Trump took office.
Mr Trump injected a fresh impetus into the Senate investigation after he himself met the Russian ambassador and foreign minister in the White House earlier this month, only a day after firing the director of the FBI.
The US president said in that encounter that he had just fired the FBI director because he was a "real nut job" and his dismissal eased "a great pressure because of Russia", the New York Times reported.
During the Oval Office chat, which US media were not invited to cover, Mr Trump also reportedly divulged secret information on the military campaign against so-called Islamic State.
Israel was reportedly the source of that sensitive intelligence.
But while in Jerusalem on Monday, Mr Trump told reporters repeatedly that he "never mentioned the word Israel" in his meeting with Russian officials.
Former FBI director Robert Mueller was appointed last week as special counsel to lead the FBI investigation following Mr Trump's firing of the law enforcement agency's director, James Comey.
Source : http://www.bbc.com/news/world-us-canada-40004300
